WASHINGTON — America, and the world, just found out what “America First” means.

President Trump could have used his inaugural address to define one of the touchstone phrases of his campaign in the most inclusive way, arguing, as did many of his predecessors, that as the world’s greatest superpower rises, its partners will also prosper.

Instead, he chose a dark, hard-line alternative, one that appeared to herald the end of a 70-year American experiment to shape a world that would be eager to follow its lead. In Mr. Trump’s vision, America’s new strategy is to win every transaction and confrontation. Gone are the days, he said, when America extended its defensive umbrella without compensation, or spent billions to try to lift the fortune of foreign nations, with no easy-to-measure strategic benefits for the United States.

“From this day forward, it’s going to be only America first,” he said, in a line that resonated around the world as soon as he uttered it from the steps of the Capitol. “We must protect our borders from the ravages of other countries making our products, stealing our companies and destroying our jobs.”

The United States, he said, will no longer subsidize “the armies of other countries while allowing for the very sad depletion of our military.”