On September 1, 1939, the German army (Wehrmacht) invaded neighboring Poland. To justify the invasion, Hitler claimed the attack was a defensive act, citing a false claim that the Poles were persecuting ethnic Germans. Additionally, the SS, in collusion with the German army, staged a phony Polish attack on a German radio station. Hitler used this phony attack to launch a retaliatory campaign against Poland.
Nazi Germany had already annexed Austria and Czechoslovakia in 1938. Hitler believed the invasion of Poland was an important step in achieving the goal of Lebensraum or “living space” for ‘Aryan’ Germans. The signing of the Molotov-Ribbentrop Pact on August 23, 1939, meant that a German invasion of Poland would not trigger a war with the Soviet Union. The Pact, a treaty of nonaggression between Germany and the Soviet Union, outlined the partitioning of Poland between Germany and the Soviet Union.
Initially the invasion was to take place on August 26, 1939. However, Hitler cancelled the invasion after Poland signed a military alliance with Britain. The alliance provided Poland with British military aid, should Germany attack. Additionally, Germany’s ally, Mussolini, declared that Italy was not ready to go to war.
When the German invasion of Poland began early on the morning of September 1, 1939, the German military overwhelmed Polish forces. Britain and France declared war on Germany on September 3, 1939. Their focus turned to fortifying their own strength in order to fend off future German offensive action. In accordance with the Molotov-Ribbentrop Pact, the Soviet Union invaded western Poland on September 17, 1939. Warsaw surrendered on September 28, 1939.