The event that marked the official beginning of the Cold War between the U.S. and the USSR is generally considered to be the announcement of the Truman Doctrine in 1947. This doctrine articulated the U.S. commitment to containing communism and providing support to countries resisting Soviet influence. It was a response to geopolitical changes in Europe after World War II, including the growing dominance of the USSR and increased tensions exemplified by events like the Berlin Blockade. This moment established the ideological and political rift between the two superpowers, setting the stage for decades of conflict and competition.