Without a doubt, the development of nuclear weapons is one application of nuclear science that has had a significant global influence. Following the observation of fission products of uranium by Hahn and Strassmann in 1938, a uranium fission weapon became possible in the eyes of a number of nuclear scientists. It was Albert Einstein who signed a letter to Franklin Roosevelt, President of the United States, and alerted him to the potential development of a nuclear weapon. Thus began the Manhattan Project, resulting in production of the first nuclear weapons.

In 1945, a bomb using the fission of ²³⁵U was dropped on Hiroshima, while a bomb using the fission of ²³⁹Pu was dropped on Nagasaki. The awesome destruction that was produced by two nuclear devices changed the face of warfare forever. Not only did these two explosions end the most destructive war in history, but the possessor of these weapons of mass destruction seemed invincible to any adversary.

The legacy of nuclear weapons remains in the soil around us, where we can still measure minute amounts of fallout from the atmospheric nuclear tests of the 1950s and 1960s. The weapons need constant maintenance because radiation damage affects the materials and triggering devices that must act together to make the weapon work. Furthermore, the isotope tritium, which is used in weapons, naturally decays. Storage and disposal of the weapons is technically difficult but probably a manageable problem. However, it is a daunting political and social problem. It is a challenge to maintain these weapons, to significantly reduce their numbers, to clean up the waste, and to insure that these weapons never will be used again.

Nuclear weapons are with us to this day and could be with us in the future. Even if all conflicts among nations end, some nuclear weapons might be retained. Earth is vulnerable to impacts from comets and asteroids. Some scientists have proposed that nuclear weapons could possibly be used to deflect them from hitting our planet.