90 years ago, while waiting to cross the road at Russell Square, the refugee physicist Leo Szilard had a thought that could unleash the awesome power locked inside the nucleus of an atom. Seven years later Otto Frisch and Rudolf Peierls wrote a secret memo to the British Government outlining the characteristics of a “super-bomb” based on Szilard’s idea. That memo led to the establishment of a top secret programme that would soon cross the Atlantic and build the world’s first nuclear weapon. One physicist at the heart of that project, Sir Józef Rotblat, had the moral courage to walk away from that research and spawn a movement of scientists that would play a central role in tempering, and ultimately reversing the nuclear arms race during the Cold War. Today, those achievements are under threat like never before and we are again called upon to warn humanity of the special dangers posed by nuclear weapons. We will explain what these weapons are, how physicists and others have mobilized to contain them, and the new challenge to prevent their use in a multipolar world where the risk of proliferation is increasing and current nuclear-armed states are modernizing their arsenals with little regard to the new threats posed to or by these weapons.