Léger's career was interrupted in 1914, when he was mobilized to serve in the French Army for two years during World War I. While at the front in Argonne, Léger sketched artillery equipment, airplanes, and fellow soldiers. This stark reality jolted him from the abstract art that had marked his style before the war, and when the war ended, Léger began his so-called "mechanical period," concentrating on realistic, orderly urban scenes celebrating workers, circuses, picnics, and everyday objects.
In the 1950s Léger began to explore mosaics, stained glass, ceramics and tapestry, and their relationship to architecture. He received many commissions to create art in architectural settings, including a pair of murals in the General Assembly Hall of the United Nations headquarters in New York, a mosaic for the Church of Notre-Dame de Toute Grace, ten stained glass windows for the church at Courfaivre, Switzerland as well as stained glass windows for the University of Caracas in Venezuela.
Léger died in 1955, but his work can be found today in major museums around the world. The Fernand Léger Museum in Biot, France, traces the artist's development from 1905 through 1955.