The Invasion of Poland, 1939 (in Poland also "the September Campaign," "Kampania wrześniowa," and "the 1939 Defensive War," "Wojna obronna 1939 roku"; in Germany, "the Poland Campaign," "Polenfeldzug," codenamed "Fall Weiss," "Case White," by the German General Staff, and sometimes called "the Polish-German War of 1939"), which precipitated World War II, was carried out by Nazi Germany, the Soviet Union and a small German-allied Slovak contingent.
The invasion of Poland marked the start of World War II in Europe as Poland's western allies, the United Kingdom, Australia and New Zealand By October 1, Germany and the Soviet Union had completely overrun Poland, although the Polish government never surrendered. In addition, Poland's remaining land and air forces were evacuated to neighboring Romania and Hungary. Many of the exiles subsequently joined the recreated Polish Army in allied France, French-mandated Syria, and the United Kingdom.
In the aftermath of the September Campaign, a resistance movement was formed. Poland's fighting forces continued to contribute to Allied military operations and did so throughout the duration of World War II. Germany captured the Soviet-occupied areas of Poland when it invaded the Soviet Union on June 22, 1941, and lost the territory in 1944 to an advancing Red Army. Over the course of the war, Poland lost over 20% of its pre-war population under an occupation that marked the end of the Second Polish Republic.

Opposing forces
Germany had a significant numerical advantage over the Polish and had developed a significant military prior to the conflict. The Heer (Army) had some 2,400 tanks organized into six panzer divisions, utilizing a new operational doctrine. It held that these divisions should act in coordination with other elements of the military, punching holes in the enemy line and isolating selected enemy units which would be encircled and destroyed. This would be repeated and followed up by less mobile mechanized infantry and foot soldiers. The Luftwaffe (Air Force) provided both tactical and strategic air power, particularly dive bombers that attacked and disrupted the enemy's supply and communications lines. Together the new operational methods were nicknamed Blitzkrieg (lightning war). Historian Basil Liddell Hart and A. J. P. Taylor conclude "Poland was a full demonstration of the Blitzkrieg theory". Due to its participation in the Spanish Civil War, the Luftwaffe was probably the most experienced, best trained and well equipped air force in the world in 1939.

Germany
Between 1936 and 1939, Poland invested heavily in industrialization in the Central Industrial Region. Preparations for a defensive war with Germany were ongoing for many years, but most plans assumed fighting would not begin before 1942. To raise funds for industrial development, Poland was selling much of the modern equipment it produced. In 1936 a National Defence Fund was set up collect funds necessary for improving fighting ability of the Polish Armed forces. The Polish Army had about a million soldiers but less than half were mobilized by September 1. Latecomers sustained significant casualties when public transport became targets of the Luftwaffe. The Polish military had fewer armoured forces than the Germans, and these units, being dispersed within the infantry, were unable to effectively engage the enemy.
Experiences in the Polish-Soviet War shaped Polish Army organisational and operational doctrine. Unlike the trench warfare of the First World War, the Polish-Soviet War was a conflict in which the cavalry's mobility played a decisive role. Poland acknowledged the benefits of mobility but was unwilling to invest heavily in many of the expensive and unproven new inventions since then and make these additions to its armed forces. In spite of this, Polish Cavalry brigades were used as a mobile mounted infantry and had some successes against both German infantry and German cavalry.
The Polish Air Force (Lotnictwo Wojskowe) was at a severe disadvantage against the German Luftwaffe although it was not destroyed on the ground. Although the Polish Air Force lacked modern fighter aircraft, its pilots were among the world's best-trained, a fact that was proven a year later in the Battle of Britain, in which the Poles played a major part in beating the Luftwaffe.
Overall, the Germans enjoyed numerical and qualitative aircraft superiority. Poland had only about 600 modern aircraft. The Polish Air Force had about 185 PZL P.11 and some 95 PZL P.7 fighters, 175 PZL.23 Karaś B, 35 Karaś A, and by September over 100 PZL.37 Łoś were produced. Additionally there were over a thousand obsolete transport, reconnaissance and training aircraft. However for the September Campaign only some 70% those aircraft were mobilised. Only 36 PZL.37 Łoś bomber aircraft were deployed for action. All those aircraft were of indigenous Polish design, with the bombers being more modern than fighters according to the Ludomil Rayski air force expansion plan relying on the strong bomber force. Polish fighter aircraft were a generation older than their German counterparts. The Polish PZL P.11 fighter, produced in the early 1930s, was capable of only 365 km/h (about 220 mi/hr), far less than German bombers; to compensate, the pilots relied on the P-11 maneuvrability and high diving speed.
The Polish Navy was a small fleet comprising of destroyers, submarines and smaller support vessels. Most Polish surface units followed Operation Peking, leaving Polish ports on August 20 and escaping to the North Sea to join with the British Royal Navy. Submarine forces participated in Operation Worek, with the goal of engaging and damaging German shipping in the Baltic Sea, but they had much less success. In addition, many Polish Merchant Marine ships joined the British merchant fleet and took part in wartime convoys.
The tank force consited of two armoured brigades, four independent tank battalions and some 30 companies of TKS tankettes attached to infantry divisions and cavalry brigades.

Poland

For more details on this topic, see Soviet order of battle for invasion of Poland in 1939. Soviet Union

For more details on this topic, see Slovak invasion of Poland. Slovakia
Order of battle of Poland:
Order of battle of invading forces:

Polish army order of battle in 1939
Polish Air Force order of battle in 1939
Polish Navy order of battle in 1939
Polish armaments 1939-1945
German order of battle for Operation Fall Weiss
Soviet order of battle for invasion of Poland in 1939 Order of battle

Main article: Causes of World War II Prelude to the campaign

Details of the campaign

Plans
The German plan Fall Weiss, for what became known as the September Campaign, was created by General Franz Halder, chief of the general staff, and directed by General Walther von Brauchitsch, the commander in chief of the upcoming campaign. The plan called for the start of hostilities before the declaration of war, which pursued a traditional doctrine of mass encirclement and the destruction of enemy forces. Germany's material advantages, including the use of modern airpower and tanks, were to be of great advantage. The infantry - far from completely mechanized but fitted with fast moving artillery and logistic support - was to be supported by German tanks and small numbers of truck-mounted infantry (the Schützen regiments, forerunners of the panzergrenadiers) to assist the rapid movement of troops and concentrate on localized parts of the enemy front, eventually isolating segments of the enemy, surrounding, and destroying them. The pre-war armored idea (which an American journalist in 1939 dubbed Blitzkrieg), which was advocated by some generals including Heinz Guderian, would have had the armor blasting holes in the enemy's front and ranging deep into the enemy's rear areas, but in actuality, the campaign in Poland would be fought along more traditional lines. This stemmed from conservatism on the part of the German high command, who mainly restricted the role of armor and mechanized forces to supporting the conventional infantry divisions.
Poland was a country well suited for mobile operations when the weather cooperated - a country of flat plains with long frontiers totalling almost 5,600 kilometres (3,500 mi), Poland had long borders with Germany on the west and north (facing East Prussia) of 2,000 kilometres (1,250 mi). Those had been extended by another 300 kilometres (500 mi) on the southern side in the aftermath of the Munich Agreement of 1938; the German incorporation of Bohemia and Moravia and creation of the German puppet state of Slovakia meant that Poland's southern flank was exposed to invasion.
German planners intended to fully utilise their advantageously long border with the great enveloping manoeuvre of Fall Weiss. German units were to invade Poland from three directions:
All three assaults were to converge on Warsaw, while the main Polish army was to be encircled and destroyed west of the Vistula. Fall Weiss was initiated on September 1, 1939, and was the first operation of the Second World War in Europe.

A main attack from the German mainland through the western Polish border. This was to be carried out by Army Group South commanded by General Gerd von Rundstedt, attacking from German Silesia and from the Moravian and Slovak border: General Johannes Blaskowitz's 8th Army was to drive eastward against Łódź; General Wilhelm List's 14th Army was to push on toward Kraków and to turn the Poles' Carpathian flank; and General Walter von Reichenau's 10th Army, in the centre with Army Group South's armour, was to deliver the decisive blow with a northwestward thrust into the heart of Poland.
A second route of attack from the northern Prussian area. General Fedor von Bock commanded Army Group North comprising General Georg von Küchler's 3rd Army, which struck southward from East Prussia, and General Günther von Kluge's 4th Army, which struck eastward across the base of the Polish Corridor.
A tertiary attack by part of Army Group South's allied Slovak units from the territory of Slovakia.
From within Poland the German minority would assist in the assault on Poland by engaging in diversion and sabotage operations through Selbstschutz units prepared before the war. German plan
The Polish defense plan, Zachód (West), was shaped by political determination to deploy forces directly at the German-Polish border, based upon London's promise to come to Warsaw's military aid in the event of invasion. Moreover, with the nation's most valuable natural resources, industry and highly populated regions near the western border (Silesia region), Polish policy centered on the protection of such regions, especially since many politicians feared that if Poland should retreat from the regions disputed by Germany (like the Polish Corridor, cause of the famous "Danzig or War" ultimatum), Britain and France would sign a separate peace treaty with Germany similar to the Munich Agreement of 1938. In addition, none of those countries specifically guaranteed Polish borders or territorial integrity. On those grounds, Poland disregarded French advice to deploy the bulk of their forces behind the natural barriers of the wide Vistula and San rivers, even though some Polish generals supported it as a better strategy. The Zachód plan did allow the Polish armies to retreat inside the country, but it was supposed to be a slow retreat behind prepared positions near rivers (Narew, Vistula and San), giving the country time to finish its mobilisation, and was to be turned into a general counteroffensive when the Western Allies would launch their own promised offensive.
The Polish Army's most pessimistic fall-back plan involved retreat behind the river San to the southeastern voivodships and their lengthy defence (the Romanian bridgehead plan). The British and French estimated that Poland should be able to defend that region for two to three months, while Poland estimated it could hold it for at least six months. This Polish plan was based around the expectation that the Western Allies would keep their end of the signed alliance treaty and quickly start an offensive of their own. However, neither the French nor the British government made plans to attack Germany while the Polish campaign was fought. In addition, they expected the war to develop into trench warfare much like World War I had, forcing the Germans to sign a peace treaty restoring Poland's borders. The Polish government, however, was not notified of this strategy and based all of its defence plans on promises of a quick relief action by their Western Allies.

Polish plan
Following several German-staged incidents (Operation Himmler), which gave German propaganda an excuse to claim that German forces were acting in self-defense, the first regular act of war took place on September 1, 1939, at 04:40, when the German Air Force (Luftwaffe) attacked the Polish town of Wieluń, destroying 75% of the city and killing close to 1,200 people, most of them civilians. Five minutes later, at 04:45, the old German battleship Schleswig-Holstein opened fire on the Polish military transit depot at Westerplatte, in the Free City of Danzig on the Baltic Sea. At 08:00, German troops, still without a formal declaration of war issued, attacked near the Polish town of Mokra; the battle of the border had begun. Later that day, the Germans opened fronts along Poland's western, southern and northern borders, while German aircraft began raids on Polish cities. Main routes of attack led eastwards from Germany proper through the western Polish border. A second route carried supporting attacks from East Prussia in the north, and a co-operative German-Slovak tertiary attack by units (Army "Bernolak") from the territory of German-allied Slovakia in the south. All three assaults converged on the Polish capital of Warsaw.
The Allied governments declared war on Germany on September 3; however, they failed to provide Poland with any meaningful support. The German-French border had a few minor skirmishes, although the majority of German forces, including eighty-five percent of their armoured forces, were engaged in Poland. Despite some Polish successes in minor border battles, German technical, operational and numerical superiority forced the Polish armies to withdraw from the borders towards Warsaw and Lwów. The Luftwaffe gained air superiority early in the campaign. By destroying communications the Luftwaffe increased the pace of the advance which overran Polish airstrips and early warning sites and causing logistical problems for the Poles. Many Polish Air Force units were now low on supplies, 98 of their number withdrew into neutral (at that time) Romania.
The Polish government (of president Ignacy Mościcki) and the high command (of Marshal Edward Rydz-Śmigły) left Warsaw in the first days of the campaign and headed southeast, arriving in Brześć on September 6. General Rydz-Śmigły ordered the Polish forces to retreat in the same direction, behind the Vistula and San rivers, beginning the preparations for the long defence of the Romanian bridgehead area.

Phase 1: German invasion

For more details on this topic, see Soviet invasion of Poland (1939). Civilian losses
At the end of the September Campaign, Poland was divided among Nazi Germany, the Soviet Union, Lithuania and Slovakia. Nazi Germany annexed parts of Poland, while the rest was governed by the so-called General Government. On September 28, another secret German-Soviet protocol modified the arrangements of August: all Lithuania was to be a Soviet sphere of influence, not a German one; but the dividing line in Poland was moved in Germany's favor, to the Bug River. Even though water barriers separated most of the spheres of interest, the Soviet and German troops met each other on numerous occasions. The most remarkable event of this kind happened in Brest-Litovsk on September 22. The German 19th panzer corps under the command of Heinz Guderian had occupied Brest-Litovsk, which lay within the Soviet sphere of interest. When the Soviet 29th Tank Brigade under the command of S. M. Krivoshein approached Brest-Litovsk, the commanders negotiated that the German troops would withdraw and the Soviet troops would enter the city saluting each other..
About 65,000 Polish troops were killed in the fighting, with 420,000 others being captured by the Germans and 240,000 more by the Soviets (for a total of 660,000 prisoners). Up to 120,000 Polish troops escaped to neutral Romania (through the Romanian Bridgehead) and Hungary, and another 20,000 escaped to Latvia and Lithuania, with the majority eventually making their way to France or Britain. Most of the Polish Navy succeeded in evacuating to Britain as well. German personnel losses were less than their enemies (~16,000 KIA).
Neither side—Germany, the Western Allies or the Soviet Union—expected that the German invasion of Poland would lead to the war that would surpass World War I in its scale and cost. It would be months before Hitler would see the futility of his peace negotiation attempts with Great Britain and France, but the culmination of combined European and Pacific conflicts would result in what was truly a "world war". Thus, what was not visible to most politicians and generals in 1939 is clear from the historical perspective: The Polish September Campaign marked the beginning of the Second World War in Europe, which combined with the Japanese invasion of China in 1937 and the Pacific War in 1941, formed the conflict known as World War II.
The invasion of Poland led to Britain and France to declare war on Germany on September 3; however, they did little to affect the outcome of the September Campaign. This lack of direct help during September 1939 led many Poles to believe that they had been betrayed by their Western allies.
On May 23, 1939, Adolf Hitler explained to his officers that the object of the aggression was not Danzig, but the need to obtain German Lebensraum and details of this concept would be later formulated in the infamous Generalplan Ost. [8] [9] The blitzkrieg decimated urban residential areas, civilians soon became indistinguishable from combatants, and the forthcoming German occupation (General Government, Reichsgau Wartheland) was one of the most brutal episodes of World War II, resulting in over 6 million Polish deaths (over 20% of the country's total population, and over 90% of its Jewish minority),-including the mass murder of 3 million Poles, regardless of religious beliefs,[10]- in extermination camps like Auschwitz, in concentation camps, and in numerous ad hoc massacres where civilians were rounded up, taken to a nearby forest, machine-gunned, and then buried, regardless of whether they were actually dead or not.
The Red Army occupied the Polish territories with mostly Ukrainian and Belarusian population. Soviets, met at the beginning as liberators by local people, shortly after started to introduce communist ideology in the area. This led to a powerful anti-Soviet resistance in the West Ukraine. Soviet occupation between 1939 and 1941 resulted in the death or deportation of least 1.8 million former Polish citizens, when all who were deemed dangerous to the communist regime were subject to sovietization, forced resettlement, imprisonment in labour camps (the Gulags) or murdered, like the Polish officers in the Katyn massacre. Part of these casualties were retributions for the attacks of the Ukrainian nationalists on the Polish villages in the West Ukraine, where vengeful feeling was particularly strong. Soviet atrocities commenced again after Poland was "liberated" by the Red Army in 1944, with events like the persecution of the Home Army soldiers and execution of its leaders (Trial of the Sixteen).

Aftermath
There are several common misconceptions regarding the Polish September Campaign:

The Polish military was so backward they fought tanks with cavalry: Although Poland had 11 cavalry brigades and its doctrine emphasized cavalry units as elite units, other armies of that time (including German and Soviet) also fielded and extensively used horse cavalry units. Polish cavalry (equipped with modern small arms and light artillery like the highly effective Bofors 37 mm antitank gun) never charged German tanks or entrenched infantry or artillery directly but usually acted as mobile infantry (like dragoons) and reconnaissance units and executed cavalry charges only in rare situations, against enemy infantry. The article about the Battle of Krojanty (when Polish cavalry were fired on by hidden armored vehicles, rather than charging them) describes how this myth originated.
The Polish air force was destroyed on the ground in the first days of the war: The Polish Air Force, though numerically inferior, was not destroyed on the ground because combat units had been moved from air bases to small camouflaged airfields shortly before the war. Only some trainers and auxiliary aircraft were destroyed on the ground on airfields. The Polish Air Force remained active in the first two weeks of the campaign, causing damage to the Luftwaffe. Many skilled Polish pilots escaped afterwards to the United Kingdom and were deployed by the RAF during the Battle of Britain. Fighting from British bases, Polish pilots were on average the most successful in shooting down German aircraft . Poland also never officially surrendered to the Germans.
The German Army used astonishing new concepts of warfare and used new technology daringly: The myth of Blitzkrieg has been dispelled by some authors, notably Matthew Cooper. Cooper writes (in The German Army 1939–1945: Its Political and Military Failure): "Throughout the Polish Campaign, the employment of the mechanised units revealed the idea that they were intended solely to ease the advance and to support the activities of the infantry…. Thus, any strategic exploitation of the armoured idea was still-born. The paralysis of command and the breakdown of morale were not made the ultimate aim of the … German ground and air forces, and were only incidental by-products of the traditional manoeuvers of rapid encirclement and of the supporting activities of the flying artillery of the Luftwaffe, both of which had as their purpose the physical destruction of the enemy troops. Such was the Vernichtungsgedanke of the Polish campaign." Vernichtungsgedanke was a strategy dating back to Frederick the Great, and was applied in the Polish Campaign little changed from the French campaigns in 1870 or 1914. The use of tanks "left much to be desired...Fear of enemy action against the flanks of the advance, fear which was to prove so disastrous to German prospects in the west in 1940 and in the Soviet Union in 1941, was present from the beginning of the war." Many early postwar histories, such as Barrie Pitt's in The Second World War (BPC Publishing 1966), attribute German victory to "enormous development in military technique which occurred between 1918 and 1940", citing that "Germany, who translated (British inter-war) theories into action… called the result Blitzkrieg." John Ellis, writing in Brute Force (Viking Penguin, 1990) asserted that "…there is considerable justice in Matthew Cooper's assertion that the panzer divisions were not given the kind of strategic (emphasis in original) mission that was to characterise authentic armoured blitzkrieg, and were almost always closely subordinated to the various mass infantry armies." Zaloga and Madej, in The Polish Campaign 1939 (Hippocrene Books, 1985), also address the subject of mythical interpretations of Blitzkrieg and the importance of other arms in the campaign. "Whilst Western accounts of the September campaign have stressed the shock value of the panzers and Stuka attacks, they have tended to underestimate the punishing effect of German artillery (emphasis added) on Polish units. Mobile and available in significant quantity, artillery shattered as many units as any other branch of the Wehrmacht." Myths

Armenian quote
History of Poland (1939–1945)
Oder-Neisse line
Polish cavalry brigade order of battle
Polish contribution to World War II
Timeline of the Polish September Campaign
Western betrayal
Blitzkrieg
Vernichtungsgedanke
War crimes of the Wehrmacht
Treatment of the Polish citizens by the occupiers Notes

Cooper, Matthew The German Army 1939-1945: Its Political and Military Failure. Stein and Day, Briarcliff Manor, NY, 19781 (ISBN 0-8128-2468-7)
Baliszewski Dariusz, Wojna sukcesów, Tygodnik "Wprost", Nr 1141 (10 October 2004), Polish, retrieved on 24 March 2005
Dariusz Baliszewski Most honoru, Tygodnik "Wprost", Nr 1138 (19 September 2004)], Polish, retrieved on 24 March 2005
Chodakiewicz, Marek Jan. Between Nazis and Soviets: Occupation Politics in Poland, 1939-1947. Lexington Books, 2004 (ISBN 0-7391-0484-5).
Ellis, John. Brute Force: Allied Strategy and Tactics in the Second World War. Viking Adult, 1st American ed edition, 1999. (ISBN 0-670-80773-7)
Fischer, Benjamin B., "The Katyn Controversy: Stalin's Killing Field", Studies in Intelligence, Winter 1999-2000, last accessed on 10 December, 2005
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Majer, Diemut et al. Non-Germans under the Third Reich: The Nazi Judicial and Administrative System in Germany and Occupied Eastern Europe, with Special Regard to Occupied Poland, 1939-1945. Johns Hopkins University Press, 2003 (ISBN 0-8018-6493-3)
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Smith, Peter C. Stuka Spearhead: The Lightning War from Poland to Dunkirk 1939-1940. Greenhill Books, 1998 (ISBN 1-85367-329-3).
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