Who exactly do we need first use against?

The Russian newspaper Izvestia has the answer to your question today in a lead article quoting the president of the Russian Academy of Geopolitical Problems, General Leonid Ivashov, who is quoted as saying the US and NATO are preparing the ground for a nuclear strike against objectionable countries.

Ivashov says:

Ivashov says that neither Russia, nor China and not anyone else is prepared to give a nuclear response to a nuclear strike by the United States against some country it doesn't like. He says the US and its allies know that and are betting on getting off scot free if they use nuclear weapons first.

He says:

That is probably correct.

Izvestia