Final answer:
Hitler wrongly presumed England and France would not defend Poland after previous appeasements. The Nazi-Soviet Pact reinforced his confidence to invade, leading to the start of World War II.
Step-by-step explanation:
Hitler invaded Poland after calculating that England and France, having sold out Czechoslovakia, would not fight to defend Poland. This assumption proved incorrect as both countries had pledged to support Poland against German aggression. The Nazi-Soviet Pact, however, gave Hitler the confidence he needed to proceed with his invasion, as it ensured that the Soviet Union would not interfere and would themselves invade Poland from the east, essentially dividing the nation between the two powers. Despite the guarantees provided to Poland by Western allies, their actual military support was non-existent when Poland faced the combined onslaught of German and Soviet forces, leading to a rapid Polish defeat and the onset of World War II.