Nuclear weapons

The most destructive weapons ever made: a single warhead can destroy a city. Nine countries are believed to possess them; Russia and the USA hold close to 90% of all warheads.
Most nuclear powers rely on a 'triad' — land-based missiles, missile submarines and bombers — so that no surprise attack can remove their ability to respond. This idea of deterrence, together with treaties such as the NPT, is why nuclear weapons have not been used in war since 1945.