The Polish September Campaign (the German attack of Poland in September 1939 - known in Polish also as the Defense War of 1939) was the military operation which started World War II.
After staging a number of false provocations (Operation Himmler), on September 1, 1939, 04:45 local time, the German battleship Schleswig-Holstein began taking the Polish enclave Westerplatte in Gdansk by the Baltic Sea under fire. Soon, German troops attacked Poland alongside its Western, Southern and Northern borders, while German aircraft started raids on Polish cities. Despite some Polish successes in minor border battles, the German technical and numerical superiority made the Polish armies withdraw towards Warsaw and Lwow. The largest battle during this campaign took place near the Bzura river west of Warsaw from September 9 to September 18 - it was the Polish attempt at a counterattack, that failed after an initial success. Warsaw, under heavy aerial bombardment from the first hours of the war, was first attacked on September 9, then got under siege from September 13 until its capitulation on September 28.
Until October 2 lasted a defence of the Hel peninsula on the Baltic Sea. The capitulation of the town of Kock (near Lublin) on October 6, after the 4-day battle, marked the end of the September Campaign.
The September Campaign was codenamed Fall Weiß ("Case White") by the German Wehrmacht. Polish historians call it Wojna obronna 1939 ("Defense War of 1939"). Tanks and aircraft (particularly fighters and ground attack aircraft like the famous Junkers Ju 87 Stuka) played a major role in the fighting. Bomber aircraft also attacked whole cities (Warsaw, for instance) causing huge losses amongst the civilian population.
About 65,000 of the Polish troops were killed, several hundred thousands were captured by the Germans or Soviets. A number of the Polish troops withdrew to neutral Romania and Hungary, from where most escaped to France or Great Britain.
There are some common myths about the Polish Campaign. Although Poland had 11 Cavalry Brigades, the Polish cavalry never charged on German tanks. Secondly, the Polish airforce, though obsolete, was not destroyed on airfields, and remained active in the first two weeks of the campaign, causing some harm to the Germans.
United Kingdom and France declared war on Germany on 3 September, but were unable to offer any real aid to their ally. Poland, fulfiling her alliance with them, had not surrendered in 1939 - there was Polish Government-in-Exile and underground civil authorities, legal successors to pre-1939 government and the Poles continued to be an extremely restive population under Nazi rule.