Vladimir Putin ratcheted up the threat of a nuclear attack if US-supplied longer range missiles are fired by Ukraine into Russia. The Russian president 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.

Russian President Vladimir Putin signed a revised nuclear doctrine on Tuesday to effectively lower his country's threshold for using nuclear weapons. Why it matters: The new doctrine suggests Russia could use nuclear weapons to respond to attacks by Ukraine carried out with American support.