Final answer:
Hitler targeted the Soviet Union for invasion to secure Lebensraum, launching Operation Barbarossa with the largest military force ever assembled, marking a pivotal turn in World War II.
Step-by-step explanation:
Hitler's strategic focus in his quest for world domination ultimately centered on the invasion of the Soviet Union. Disregarding the German-Soviet Nonaggression Pact, Hitler launched Operation Barbarossa on June 22, 1941, assembling the largest invasion force in history to date, with over three million troops from various allied nations. His ideological drive for Lebensraum, or living space, aimed at conquering Eastern Europe and subjugating Slavic peoples to German rule. Although initial rapid advancements were made, the sheer size of the Soviet Union, along with the harsh winter and fierce Soviet resistance, led to the eventual failure of the operation and marked a significant turning point in World War II.