I Was Developed During The Manhattan Project And I Was Conceived By Einstein

Riddle: I was developed during the Manhattan Project. I was conceived by Einstein, and built by a team led by Robert Oppenheimer, in Los Alamos, New Mexico. I was built in 1942. The U.S. was the first to develop me, with the help of German, Italian, and American scientists. I work by converting mass to energy, by splitting Uranium with Neutrons, which creates fission. This creates an explosion. My explosion is the release of nuclear energy. There are four parts to my explosion 1) the blast wave, 2) the thermal radiation, 3) initial nuclear radiation, and 4) residual nuclear radiation. As a single explosion, I destroyed more lives and property and injured more people, than any other explosion during all of World War II. What am I?