In 1939, people studying physics began to understand the theory of nuclear fission weapons, but no country knew how to build one. When World War II started, Germany, the United Kingdom and the United States wanted to build nuclear weapons. Germany did not finish building them, partly because many of the best people studying physics fled Germany after Nazi rule started. The United Kingdom started working in 1939, but it took too much money, so they gave up in 1942. Later that year, the United States started a very large program to build nuclear weapons. It built upon the work done in the United Kingdom. The program was called the "Manhattan Project". 