The idea is to launch quickly so that the counter-attack missiles will be safely off the ground before the incoming missiles rain down. That way, the promise of a swift counter-attack can serve as a credible deterrent to a first strike. Launch on warning leaves precious little time for thoughtful deliberations. Each country has submarine-based nuclear missiles within 15 minutes' striking distance of the other. Then it's over.
Mistakes are inevitable.
It was a rocket rising into the sky somewhere off the coast of Norway. Such a rocket could conceivably deliver 8 nuclear bombs to Moscow within 15 minutes, so word went out immediately throughout the Russian military command. As the various stages of the rocket separated from each other, the radar blips made it seem as if an attack by several missiles might be under way. The crisis passed. The rocket turned out to be a U.S. scientific probe intended to explore the upper atmosphere, to improve human knowledge of the northern lights.