The Manhattan Project was the code name for the secret research and development project that produced the first nuclear weapons during World War II. The project was led by the United States with participation from the United Kingdom and Canada. The Manhattan Project developed multiple designs and produced three nuclear weapons between 1942 and 1946, the first being tested at the Trinity site in New Mexico and the other two being dropped on the Japanese cities of Hiroshima and Nagasaki in August 1945.