With Donald J. Trump’s sweeping election victory Tuesday, the world is now 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 the assurance of American protection for democratic partners.

Despite a lack of substantive foreign policy debate in 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 made clear that he intends 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 such as 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 Mr. Trump’s first term, the world already knows that the only certainty with him is uncertainty. The president-elect has often said that keeping the world guessing is his ideal foreign policy. And as the votes were counted, some officials from Asia to Latin America responded with public reassurances, stressing that elements of their relationships with the United States would not likely change.