WASHINGTON — President Biden is considering deploying several thousand U.S. troops, as well as warships and aircraft, to NATO allies in the Baltics and Eastern Europe, an expansion of American military involvement amid mounting fears of a Russian incursion into Ukraine, according to administration officials. The move would signal a major pivot for the Biden administration, which up until recently was taking a restrained stance on Ukraine, out of fear of provoking Russia into invading. But as President Vladimir V. Putin has ramped up his threatening actions toward Ukraine, and talks between American and Russian officials have failed to discourage...