Mutual Assured Destruction

Mutual assured destruction (or MAD for short) is a strategy that countries use to make sure that no one tries to start a nuclear war. In this strategy, each country has enough nuclear weapons to be able to defend itself from a nuclear attack and to also cause enough destruction that it would be extremely bad for the other country. This means that if one country attacks the other with a nuclear weapon, the other country will use their nuclear weapons to cause even more destruction, making it not worth either side to actually start a nuclear war. This is like two kids saying, "if you hit me, I'll hit you back and it will be twice as bad".