With Donald J. Trump’s sweeping election victory on Tuesday, the world is preparing for another four years of unpredictability and “America first” protectionism that could reset the ground rules of the global economy, empower autocrats and erase American protection for democratic partners.

Although foreign affairs proved largely absent from the campaign, Mr. Trump has made several statements that — if turned into policy — would transform America’s relationship with both allies and adversaries. He has pledged to end the war in Ukraine in 24 hours, a promise many assume amounts to the withdrawal of American aid for Ukraine, which would benefit Russia.

More broadly, he has vowed to make the world’s most powerful country more isolationist, more combative with tariffs, more openly hostile to immigrants, more demanding of its security partners, and less engaged on global challenges like climate change.

Many believe the impacts could be greater than anything seen since the start of the Cold War.

“It accelerates the already deep trend of an America looking inward,” said James Curran, a professor of modern history at the University of Sydney. “Allies are going to have to save the multilateral furniture while it’s still around — they have to hope that America buys back in.”

By now, after witnessing his first term, the world already knows that the only certainty with Mr. Trump is uncertainty. He has often said that keeping the world guessing is his ideal foreign policy.

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