Vasili Alexandrovich Arkhipov

In 1962 the USSR sent a nuclear armed submarine to the international waters off Cuba. The sub was located by the US Navy and depth charges were dropped to force the sub to surface. 

At depth the crew had been out of communication with Moscow for days and feared that war had broken out. 

The captain wanted to launch a nuclear torpedo. 

Launch required a 3 of 3 signoff from the captain, the political officer and Arkhipov (the executive officer, who was chief of staff of the brigade). 

Arkhipov refused to authorise the attack, and persuaded the captain to surface and await orders.

Had Arkhipov not refused, there is a high probability that the USSR would have made a nuclear strike against the US Navy, which could quickly have led to thermonuclear war which would have killed billions.