The Cuban Missile Crisis
American spy planes photographed Soviet nuclear missiles being installed in Cuba, ninety miles from Florida. For thirteen days in October 1962, Kennedy and Khrushchev traded ultimatums while the U.S. blockaded the island and both militaries went to their highest alert. It ended in a deal: the Soviets withdrew the missiles publicly, the Americans withdrew missiles from Turkey quietly, and a direct hotline was installed between the capitals. It remains the closest the world has come to deliberate nuclear war — and the closest it came by accident was during the same crisis, underwater.
A weapon or tactic upends the prevailing balance of power and renders an old defensive or offensive order obsolete.