Russian President Vladimir Putin on Tuesday 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.

Putin’s endorsement of the new nuclear deterrent policy comes on the 1,000th day after he sent troops into Ukraine on Feb. 24, 2022.

It follows U.S. President Joe Biden’s decision to let Ukraine strike targets inside Russia with U.S.-supplied longer-range missiles.

The signing of the doctrine, which says that any massive aerial attack on Russia could trigger a nuclear response, reflects Putin’s readiness to threaten use of the country’s nuclear arsenal to force the West to back down as Moscow presses a slow-moving offensive in Ukraine.