Among global elites, Donald J. Trump’s recent phone call with Taiwan’s president has induced fear on a scale seldom matched since Ronald Reagan’s “Evil Empire” speech. The Sydney Morning Herald warned that the phone call “risks provoking a cold war between the United States and China with potentially catastrophic economic and security implications.” The fright appears to confirm the narrative formed earlier this year by headlines like “Donald Trump Terrifies World Leaders.”

The fear is real. Mr. Trump has indeed terrified foreign leaders with his “America first” mantra, his promises to enlarge the American military and his tough talk on everything from the Islamic State to Air Force One. The good news is that his administration can turn this fear to the benefit of the United States.

During the last eight years, President Obama showed what happens when the world’s greatest power tries strenuously to avoid giving fright. He began his presidency with lofty vows to conciliate adversaries, defer to the opinions of other countries and reduce America’s military commitments. Consequently, he received rapturous applause in European capitals and a Nobel Peace Prize. In the real world of geopolitics, however, the results have been catastrophic.

Mr. Obama’s passivity in the face of provocations and his failure to enforce the “red line” in Syria led Russia, China and other adversaries to seek new gains at America’s expense. His promises to “end the wars” in Iraq and Afghanistan satisfied the cosmopolitan chatterers of Stockholm, Paris and New York, but they deflated American allies in Baghdad and Kabul, and emboldened adversaries in Iran and Pakistan. So severe was the damage that he had to send troops back to Iraq in 2014, and had to abort his plans to withdraw all American forces from Afghanistan before leaving office.