Final answer:
Germany launched an all-out attack on Poland on September 1, 1939, leading to the start of World War II, with Poland quickly succumbing to the German Blitzkrieg despite having a mutual defense agreement with Britain.
Step-by-step explanation:
On September 1, 1939, Germany launched an all-out attack on Poland after securing a neutrality pact with the USSR. This invasion marked the beginning of World War II as it led Britain and France to declare war on Germany, thus forming the Allies. Despite the mutual defense agreement between Britain and Poland, Poland had to confront the German Wehrmacht alone, with Soviet forces soon invading from the east as part of a secret provision in their pact with Germany. The Polish defense was overwhelmed by the German strategy known as Blitzkrieg, leading to Poland's surrender in just over a month.