- 1941 Franklin D. Roosevelt takes oath on March 4; begins third term.
- 1941 Lend-Lease Act became law March 11.
- 1941 Churchill & Roosevelt develop The Atlantic Charter, Aug 14 in Argentina.
- 1941 December 7, the Day of Infamy, Japan bombed Pearl Harbor, HI.
U. S. declares war on Japan, December 8, and on Germany December 11.
- 1942 Executive Order 9066 imprisons Japanese-Americans for 3 years.
- 1942 The Battle of the Coral Sea, May 7-8.
The Battle of Midway, June 4.
The Battle of Guadalcanal: August 7.
- 1942 First self-sustaining nuclear chain reaction, by Fermi, December 2.
- 1944 Landing at Anzio Beach by the Allies, January 22.
- 1944 D-Day at Normandy! The Longest Day begins, June 6.
- 1944 The Battle of the Philippine Sea, June 19.
- 1945 Marines raise the flag on Mount Suribachi, Iwo Jima, February 23.
- 1945 Franklin D. Roosevelt takes oath on March 4; begins fourth term.
- 1945 Roosevelt dies on April 12; Harry S Truman becomes 33rd President.
- 1945 First atomic bomb exploded at Los Alamos, New Mexico, July 16.
- 1945 B-29 'Enola Gay' drops atomic bomb on Hiroshima, Aug 6.
- 1945 Last atomic bomb then in existence dropped at Nagasaki, August 9.
- 1945 World War II ends: first in Europe, May 7; then in Japan, Sept 2.
- 1945 United Nations chartered in San Francisco; in 1950 moves to NYC.
- 1946 Cold War begins as U.S.S.R. expands through eastern Europe.
Sir Winston Churchill coins term "Iron Curtain."
- 1946 Taft-Hartley Act outlaws certain practices of trade unions.
- 1947 Truman Doctrine opposes Communism in Greece, Turkey; Mar 12.
- 1947 Marshall Plan helps rebuild Europe, June 5.
- 1947 Transistor invented at Bell Labs, in New Jersey, Dec 23.
- 1948 Foreign Assistance Act funds the Marshall Plan, April 3.
- 1948 Organization of American States (OAS) formed, April 30.
- 1948 United Nations creates Republic of Israel out of Palestine.
- 1948 Berlin airlift begins nearly a year of relief to overcome blockade.
- 1948 Native Americans allowed to vote (finally) in New Mexico and Arizona.
- 1949 Harry S Truman takes oath on January 20; begins second term.
- 1949 North Atlantic Treaty Organization born, April 4.