Why did Pootin threaten the whole world with nuclear weapons, but then stopped doing so?

Yep. Mutually Assured Destruction.

Some countries will use the threat of nuclear war to get what they want. But what they want often involves still being alive afterwards. Putin, like the rest of the leaders of countries with nuclear arms, knows that a nuclear strike demands a nuclear response. There is no other option and unfortunately the whole world suffers as a result.