President Joe Biden joined an emergency NATO summit Friday to strengthen the frantic Western response to Russia’s invasion of Ukraine and fears for the security of Europe. The US president, who has spent weeks trying to lead a united trans-Atlantic response to Russia’s aggression against its neighbor, was meeting “with fellow NATO heads of state and government in an extraordinary virtual summit to discuss the security situation in and around Ukraine,” the White House said. The meeting, with Biden joining from the White House Situation Room, was not open to the media. It came as Russian troops entered parts of the capital Kyiv after assaulting Ukraine from multiple directions and Russian President Vladimir Putin called for the ouster of the country’s government. Caught between wanting to resist Russia’s blatant overturning of post-World War II European security norms and unwillingness to risk confrontation between the nuclear armed powers, NATO is walking a fine line in the face of what looks like a resurgence of the Cold War.Ukraine is not part of NATO, but four neighboring countries are and the United States has rushed troops to the eastern flank to reassure allies jittery about Putin’s broader intentions.