Nuclear weapon
In 1939, physicists 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. The United Kingdom started working in 1939 but found it so expensive that it gave up in 1942. Later that year, the United States started a very large program to build nuclear weapons. Building upon the work done in the United Kingdom, the program was called the "Manhattan Project".