TOKYO — Under normal circumstances, just one of the announcements that came out of the White House on Thursday would have unnerved American allies in Asia.

But in a single day, President Trump managed to unsettle the region on not just one front but two. Hours after he signed orders to impose stiff and sweeping tariffs on imported steel and aluminum, including from key allies like Japan and South Korea, he accepted an invitation to personally meet North Korea’s leader, Kim Jong-un, for negotiations over the North’s nuclear program.

For allies who have long looked to the United States to provide security and stability, it was a dizzying jolt of drama that injected fresh uncertainty into strategic calculations in the region, where China is seeking to supplant the United States as the major power.

“This is without question a big opportunity for China,” said Ian Bremmer, the president of the Eurasia Group, a New York-based research firm that forecasts global risks. “The United States has become a less certain partner for a while now.”