After World War II, America and the Soviet Union came out with tension as the US got all the credit for ending the war, but the Soviet Union scarified many soldiers, resources, and the Soviet Union fought from the start. After the passing of FDR Harry Truman became president and took a tougher approach and believed that the spread of Communism will domino and take the US down as well.
Timeline of the Early Cold War
1945: February 4-11 - Yalta Conference
1945: August 6 - United States first used atomic bomb in war
1945: August 8 - Russia enters war against Japan
1945: August 14 - Japanese surrenders, ending World War II
1946: March - Winston Churchill delivers "Iron Curtain" speech
1947: March - Truman announces Truman Doctrine
1947: June - Marshall Plan is announced
1948: February - Communist takeover in Czechoslovakia
1948: June 24 - Berlin blockade begins
1949: July - NATO treaty ratified
1949: May 12 - Berlin Blockade ends
1949: September - Mao Zedong, a communist, takes control of China
1949: September - Soviets explode first atomic bomb
1955: May – Warsaw Pact