Thousands of U.S. troops were put on standby to deploy to eastern Europe as fears of a Russian ground invasion into neighboring Ukraine looms over the European continent.

President Joe Biden’s decision to alert the military units on Monday represents an abrupt change in approach to the crisis as tensions worsen along the Ukrainian border. For weeks, the Biden Administration has restrained from mobilizing military forces as it sought to resolve the situation with Moscow diplomatically. But the lack of progress—and continued build-up of Russian forces—has prompted Biden reevaluate the U.S. options, say administration officials.

China flew 39 warplanes toward Taiwan  in its largest such sortie of the new year, amid tensions over the self-ruled island's future and as the U.S. pushes to assert its presence in the region.

The Chinese formation Sunday night included 24 J-16 fighter jets and 10 J-10 jets, among other support and electronic warfare aircraft, according to Taiwan's defense ministry.

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