Educated in the United States and deeply pro-American, Finland’s president-elect, Alexander Stubb, looked perfectly poised to lead his nation into a stronger trans-Atlantic partnership and redefine its role in the global order as a newly minted NATO member.

Instead, he will enter office next month at a time when U.S. politics has once again thrown the durability of that relationship — and the wisdom of European nations counting on it — into question.

For weeks, the two candidates in Finland’s runoff presidential elections, which Mr. Stubb won on Sunday, had played up their pro-NATO credentials and tough views on Russia. Then the former U.S. president Donald J. Trump threatened that, if re-elected, he would let Russia “do whatever the hell they want” against NATO allies that do not contribute sufficiently to collective defense.

That is hardly what this tiny Nordic nation of 5.6 million, after decades maintaining a policy of nonalignment, wants to hear, now that it holds NATO’s longest border with Russia — and as European leaders warn that the continent’s confrontation with Moscow may drag on for decades.

Mr. Trump’s comments have been a harsh reminder to many European nations that banking on Washington in the face of Russia’s invasion of Ukraine is no longer as sure a bet as it seemed.