The division of the world after World War II was primarily between two superpowers, the United States of America and the Union of Soviet Socialist Republics. This division was a result of the ideological differences between capitalism and communism, which intensified during the Cold War period.

The division of the world into the blocks of USA and USSR can be explained in the following points:

1. The Yalta Conference: In February 1945, the leaders of the Allied Powers, Winston Churchill, Franklin D. Roosevelt, and Joseph Stalin met at Yalta to discuss the reorganization of Europe after the war. They agreed to divide Germany into four zones of occupation, with each zone controlled by one of the Allied Powers- United States, Great Britain, France, and the Soviet Union.

2. The Truman Doctrine: In March 1947, President Harry Truman declared that the United States would provide military and economic aid to any country threatened by communism. This was a clear indication that the United States was committed to containing the spread of communism and limiting the influence of the Soviet Union.

3. The Marshall Plan: In June 1947, the United States launched the Marshall Plan, which provided economic assistance to Western European countries to help them rebuild after the war. The Soviet Union saw this as an attempt by the United States to extend its influence and increase its power in Europe.

4. The formation of NATO: In April 1949, the United States, Canada, and several Western European countries formed the North Atlantic Treaty Organization (NATO) to provide collective defense against the Soviet Union. This was seen as a direct threat to the Soviet Union, and it responded by forming the Warsaw Pact with its satellite states in Eastern Europe.

5. The Korean War: In 1950, North Korea, which was supported by the Soviet Union and China, invaded South Korea. The United States intervened to support South Korea, leading to a three-year conflict that ended in a stalemate. This conflict heightened tensions between the United States and the Soviet Union and showed the world that the two superpowers were willing to engage in proxy wars to advance their interests.

In conclusion, the world was divided into the blocks of USA and USSR after World War II due to the ideological differences between capitalism and communism. This division led to the Cold War period, which lasted until the collapse of the Soviet Union in 1991.