The right answer is the development of the atomic bomb.
The full-scale development of a Nuclear bomb came after the Japanese attack in Pearl Harbor, and knowing Germany was leading nuclear uranium research. The project Manhattan was created. The project joined the most renowned scientist of the century with industry, military and thousands of ordinary Americans working at localities across the country to interpret original scientific discoveries into an entirely new kind of weapon, the Atomic Bomb. At its peak, the project employed 130,000 workers and, by the end of the war, had spent $2.2 billion.