In the weeks after President Emmanuel Macron called a snap election last summer that resulted in a deeply divided French Parliament, if his name came up it was often to call for his resignation.

The unpopular president, long derided by critics as aloof, all-controlling and arrogant, looked certain to ride out the final three years of his term as a lame duck atop an unstable government of his own creation, with a rotating cast of prime ministers, and little to show for it.

But President Trump has changed that. The American leader has abruptly reversed 80 years of friendly policy toward Europe, withdrawing support for Ukraine and siding with Russia, leaving European leaders panicked and lost. In doing so, he has made this Mr. Macron’s moment.

The French president, who once seemed on the verge of disappearing, is now in the headlines daily. Mr. Macron has gathered European leaders repeatedly in Paris, rushed to Washington and later to London, and generally become the focal point of Europe’s struggling effort to stand on its own feet.

After years of warning of the “imminent brain death” of NATO, Mr. Macron’s admonition now seem prescient as Mr. Trump threatens to turn his back on the alliance.

Mr. Macron’s talk of European boots on the ground to help keep the peace in Ukraine, rejected not long ago as impossible by incredulous allies, is now a plan being worked through as a plausible way to stem the fighting.