The invasion of Poland in 1939 (also known in Poland as the "1939 Defensive War" (Wojna obronna 1939 roku), in Germany as the "Poland Campaign" (Polenfeldzug), codenamed Fall Weiss ("Case White") by the German General Staff, and in some places the "Polish September Campaign" or "Polish-German War of 1939"), was carried out by Nazi Germany, the Soviet Union and a small German-allied Slovak contingent which precipitated the onset of World War II. The invasion of Poland marked the start of World War II in Europe as Poland's western allies, the United Kingdom and France, declared war on Germany on September 3. The campaign began on September 1, 1939, one week after the signing of the secret Molotov-Ribbentrop Pact, and ended on October 6, 1939, with Germany and the Soviet Union occupying the entirety of Poland.

Following a German-staged "Polish attack" on 31 August 1939, the following day German forces invaded Poland from the north, south, and west. Spread thin defending their long borders, the Polish armies were soon forced to withdraw east. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage. Polish forces then began a withdrawal southeast, following a plan that called for a long defence in the Romanian bridgehead area where the Polish forces were to await an expected Allied counter-attack and relief.

On September 17, 1939, the Soviet Red Army invaded the eastern regions of Poland in cooperation with Germany. The Soviets were carrying out their part of the secret appendix of the Molotov-Ribbentrop Pact, which divided Eastern Europe into Nazi and Soviet spheres of influence. With the unexpected Soviet invasion, the Polish government decided the defence of the Romanian bridgehead was no longer feasible and ordered the evacuation of all troops to neutral Romania. By 1 October, 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.

Germany

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 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), but historians generally hold that German operations during the campaign were conservative, owing more to traditional methods. The strategy of the Wehrmacht (Armed Forces) was more in line with Vernichtungsgedanken, or a focus on envelopment to create pockets in broad-front annihilation.

Aircraft played a major role in the campaign. Bomber aircraft also attacked cities, causing huge losses amongst the civilian population through terror bombing. The Luftwaffe forces consisted of 1,180 fighter aircraft: 290 Ju 87 Stuka dive bombers, 290 conventional bombers (mainly of the He 111 type), and an assortment of 240 naval aircraft. In total, Germany had close to 3,000 aircraft, with nearly two thirds of them up to modern standards. Half of these were deployed on the Polish front. The Luftwaffe was among the best trained and equipped air forces in 1939.

Poland

Between 1936 and 1939, Poland invested heavily in industrialization in the Central Industrial Region (Centralny Okręg Przemysłowy). 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. The Polish Army had about a million soldiers but less than half were mobilised by 1 September. 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 was at a severe disadvantage against the German Luftwaffe although, contrary to popular belief, it was not destroyed on the ground. Although the Polish Air Force lacked modern fighter aircraft, its pilots were also among the world's best-trained.

Overall, the Germans enjoyed numerical and qualitative aircraft superiority. Poland had only about 400 aircraft, including 169 fighters and some obsolete transport, reconnaissance and training aircraft. Only 36 Polish aircraft could be considered modern, such as the PZL.37 Łoś bomber. The other Polish craft were far older than their German counterparts. The Polish PZL P.11 fighter, produced in the early 1930s, was capable of only 350 km/h (about 210 mi/hr), far less than German bombers.

The Polish Navy was a small fleet comprising 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 with much less success. In addition, many Polish Merchant Marine ships joined the British merchant fleet and took part in wartime convoys.

Aftermath

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. At Brest-Litovsk, Soviet and German commanders held a joint victory parade before German forces withdrew westward behind a new demarcation line.

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 (grand total 680,000 prisoner). 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), but the loss of approximately 30% of armored vehicles during the campaign was one of the reasons the plans for an immediate attack west were discarded.

 Soviet (left) and German officers meet after the Soviets' invasion of Poland.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 would form the conflict known as World War II.

The invasion of Poland led to Britain and France declaring 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.

 Survivor of bombing of Warsaw.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. 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), including the mass murder of 3 million Polish Jews in extermination camps like Auschwitz. Red Army occupied the Polish territories with mostly Ukrainian and Belorussian population. Soviets, met at the beginning as liberators by local people, shortly after started to introduce communist ideology in the area. Badly adapted by the population, 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 simply murdered, like the Polish officers in the Katyn massacre. Part of these casualties were due to 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).