Final answer:
The Yalta Conference between Allied leaders marked the beginning of the Cold War.
Step-by-step explanation:
The event that marked the beginning of the Cold War was the Yalta Conference between Allied leaders. At this conference in February 1945, Stalin, Roosevelt, and Churchill met to discuss the postwar world and determine the division of Germany. The agreement made at Yalta resulted in the Soviet military occupation of Eastern Europe, which sparked the Cold War between the United States and the Soviet Union.