The pall of war hung over the 1939 World's Fair. Czechoslovakia and Poland fell to German aggression in that year, Italy invaded Albania, and the Japanese expanded their Pacific empire. The United States entered the war after the Japanese attacked Pearl Harbor on December 7, 1941. In New York, security was increased on all bridges, tunnels, factories, and other significant points.

During the war, the lights of Broadway and the spectacular New York City skyline went dark or were dimmed. While the mayor moved north into Gracie Mansion (originally built in 1804 and redesigned by Robert Moses), the people of the city watched and waited for news from overseas.

The jubilant celebrations that swept through the five boroughs on V-J Day were immortalized in one photograph of a sailor and nurse kissing in Times Square. After the city welcomed its war heroes home with a rousing parade, New York continued the building expansion of the prewar era.

Among the new construction projects was a massive complex along the East River. After World War II, the United States invited the newly created United Nations to build its headquarters in an American city. The U.N. considered a number of possibilities before settling on New York. John D. Rockefeller donated money to purchase the site, and the city of New York gave more land to the U.N. as a gift. The 18 acres that comprise the U.N. complex are considered international land.