It started with one explosion in 1945. An explosion unlike any the world had ever seen. The first nuclear weapon, detonated by the United States, launched an era of nuclear proliferation that persists to this day. With the end of the Cold War, nuclear weapons are increasingly irrelevant, but the threat they represent is still very real. The tide of proliferation has ebbed. Now we have the chance to roll it back to end the threat of nuclear weapons forever.