Vannevar Bush put together a hard-working team. And in the years that followed, American scientists and engineers developed one invention after another to help the war effort.

Scientists developed new devices to help the Navy find German submarines. They improved methods for bombers to find their targets. And they developed more powerful rockets to protect American troops when they landed on foreign beaches.

American scientists and doctors also made great progress in improving the methods of wartime medicine. World War Two may well have been the first war in history in which a wounded soldier was more likely to survive than to die.

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But, in many ways, the most important scientific development of the period was the atomic bomb.

In nineteen thirty-nine, Albert Einstein wrote President Roosevelt a letter. The scientist told the president that it might soon be possible to build a powerful weapon -- a weapon that would use the power of the atom. And he urged Roosevelt to get American scientists to build the atomic bomb before German scientists could build one.

Roosevelt agreed. He created a special team of scientists. Their work became known as the Manhattan Project. Roosevelt made sure that these scientists got all the money and supplies they needed.

Roosevelt died before the scientists could complete their work. But in April nineteen forty-five, the scientists told the new president, Harry Truman, that they were almost ready to test their invention. Just three months later, they exploded the world's first atomic bomb in the desert in the southwestern state of New Mexico.