 The undertaking would be of an unimaginable scale, and the resources required to engineer for such use as a nuclear bomb, and nuclear power were deemed too great to pursue.At the urging of Albert Einstein and Leo Szilard in their letter of August 2, 1939, the United States — in collaboration with Britain and Canada — recognized the potential significance of an atomic bomb.They embarked in 1942 upon work to achieve a usable device.