After the war, the men returned, having seen the rest of the world. No longer was the family farm an ideal; no longer would blacks accept lesser status. The GI Bill allowed more men than ever before to get a college education. Women had to give up their jobs to the returning men, but they had tasted independence.

The forties are pretty well defined by World War II. US isolationism was shattered by the Japanese bombing of Pearl Harbor. As President Franklin D. Roosevelt guided the country on the homefront, Dwight D. Eisenhower commanded the troops in Europe. Gen. Douglas MacArthur and Adm. Chester Nimitz led them in the Pacific. The successful use of penicillin by 1941 revolutionized medicine. Developed first to help the military personnel survive war wounds, it also helped increase survival rates for surgery. The first eye bank was established at New York Hospital in 1944.  Unemployment almost disappeared, as most men were drafted and sent off to war.  The government reclassified 55% of their jobs, allowing women and blacks to fill them. First, single women were actively recruited to the workforce. In 1943, with virtually all the single women employed, married women were allowed to work. Japanese immigrants and their descendants, suspected of loyalty to their homelands, were sent to internment camps.

There were scrap drives for steel, tin, paper and rubber.  These were a source of supplies and gave people a means of supporting the war effort. Automobile production ceased in 1942, and rationing of food supplies began in 1943. Victory gardens were re-instituted and supplied 40% of the vegetables consumed on the home front. In April, 1945, FDR died, and President Harry Truman celebrated V-E Day on May 8, 1945. Japan surrendered only after two atomic bombs were dropped on Hiroshima and Nagasaki. The United States emerged from World War II as a world superpower, challenged only by the USSR. While the USSR subjugated the defeated countries, the US implemented the Marshall Plan, helping war-torn countries to rebuild and rejoin the world economy. Disputes over ideology and control led to the Cold War. Communism was treated as a contagious disease, and anyone who had contact with it was under suspicion. Alger Hiss, a former hero of the New Deal, was indicted as a traitor and the House Un-American Activities Committee began its infamous hearings.

Returning GI's created the baby boom, which is still having repercussions on American society today. Although there were rumors, it was only after the war ended that Americans learned the extent of the Holocaust. Realization of the power of prejudice helped lead to Civil Rights reforms over the next three decades. The Servicemen's Readjustment Act, commonly known as the GI Bill of Rights, entitled returning soldiers to a college education. In 1949, three times as many college degrees were conferred as in 1940.  College became available to the capable rather than the privileged few.

Television made its debut at the 1939 World Fair, but the war interrupted further development. In 1947, commercial television with 13 stations became available to the public. Computers were developed during the early forties. The digital computer, named ENIAC, weighing 30 tons and standing two stories high, was completed in 1945.