The Cold War kept from getting hot in no small part because of MAD — Mutual Assured Destruction. The idea that if one side attacked, even in a limited way, with nuclear weapons, the other would attack back with everything they had. That would mean the only viable attack for the aggressor would be an all out attack, but since that had no guarantee of destroying the ability of the other side of launching a devastating counter-attack (especially via nuclear-powered ballistic missile submarines, aka "boomers"), the attack would almost certainly result in the destruction of both sides.
So no attack ever happened (not to say we didn't come close here or there).
In fact, the system was predicated on the idea that both sides knew what the other was up to.
Since only two sides (essentially) had the bomb, the scheme was stable. Both sides were rational, with rational goals, and so could be trusted to act in a predictable manner.
Today there is news that the North Koreans have claimed to have a working bomb, and Iran is promising "burning hell" for any aggressive acts against its nuclear weapons program. Neither North Korea nor Iran could be easily labelled rational. Both are aggressive, almost rabid at times. In the case of Iran, there is the added motivation of other-worldly paradise for those who vanquish Allah's enemies — America and Israel.
Furthermore, both have a history of working with other nations or with terror groups to arm and supply them in order to improve their ability to spread terror and mayhem. What if North Korea never launches a nuclear weapon against Japan or the United States, but rather gives a warhead to a well-financed south Asian Islamic group? Iran is already the paymaster for Hizbollah — they would have no problem finding people willing to drive a warhead into an Israeli city, or sail a warhead into a Mediterranean or American port, and then detonate it.
In these scenarios, there is no launch of a missile that NORAD satellites can track. No way to know it was one of these countries that provided the weapon.