Chim did not work as a photojournalist during World War II. He ran a photo-finishing business in New York and then served as a photo-interpreter with the U.S. army in Europe from 1942 to 1945. In 1947 Chim, Capa, and Cartier-Bresson fulfilled their long-held desire to form a new kind of picture agency. Together with English photographer George Rodger, they founded Magnum Photos, an international photographic cooperative with editorial independence.
In 1948 Chim accepted an assignment to take photographs for a UNICEF book intended to show the horrid condition of needy children in postwar Europe and the work undertaken by the United Nations agency to ameliorate their situation. For Chim it was a perfect project. He had always liked being around children and loved to photograph them. As a staunch pacifist, he knew that nothing could speak more poignantly to the senseless destruction of war than the plight of these innocent victims.
In twelve weeks Chim traveled to his native Poland and to Hungary, Austria, Italy, and Greece. No other series better demonstrates his deeply ingrained humanitarian sympathies, and the profoundly moving photographs are widely considered to be among his best work. Forty-seven of the approximately 5,000 photographs that Chim took appear in the UNICEF book Children of Europe (1949). Select images also were published in leading magazines throughout the world, including eleven in Life's 1948 Christmas issue.