The Manhattan Project has started yet before World War II. It attempted to develop a functional atomic bomb that could be used in the war. The US feared that German scientists had been creating a similar weapon since the 1930s. Since 1939, the agencies created by President Roosevelt started working on the nuclear project. While the US prepared for the nuclear project, Japan attacked Pearl Harbor. President Roosevelt declared that the US would enter World War II to fight against Germany and Japan.
On December 28, 1942, Roosevelt eventually launched the Manhattan Project, having authorized scientists to make research on nuclear chain reactions. Testing of nuclear reactions was conducted in remote areas of New Mexico, Tennessee, and Washington. The first Manhattan Project bombs were built and tested at Los Alamos Laboratory, New Mexico, in 1943. These were uranium- and plutonium-based samples. Two years later, the first nuclear bomb was detonated also in New Mexico. The Manhattan Project was practically applied in Hiroshima and Nagasaki. As the Japanese refused to surrender, the US leveled the two Japanese cities to the ground having killed more than 100,000 people. In the end, Japan surrendered on August 14, 1945.