Russian President Vladimir Putin has signed a revised nuclear doctrine declaring that a conventional attack on Russia by any nation that is supported by a nuclear power will be considered a joint attack on his country.

President Vladimir Putin on Tuesday formally lowered the threshold for Russia’s use of its nuclear weapons, a move that follows U.S. President Joe Biden’s decision to let Ukraine strike targets inside Russian territory with American-supplied longer-range missiles.

Putin lowered the bar for a nuclear attack after the US lifted a ban on Ukraine using long-range missiles. Ukraine carried out its first strike with the weapons.