When President Biden addresses world leaders at the United Nations on Tuesday morning, his aides promise a speech filled with declarations about America’s role in shaping the future.

They say he will “reaffirm America’s leadership,” “rally global action” and provide “his vision for how the world should come together” on its most pressing challenges.

But the truth is that Mr. Biden will speak at a time of deep uncertainty about the future of America’s role in the world, including the war in Ukraine, escalating conflicts in the Middle East and growing economic competition with China.

Mr. Biden has vowed to continue pursuing a cease-fire that could end the fighting in Gaza, and his national security aides are feverishly working to forestall a broader war with Hezbollah in Lebanon. In Ukraine, Mr. Biden still faces urgent decisions, including whether to allow the use of American long-range weapons to strike deep into Russia.

And yet, there is a sense of precariousness when it comes to America’s longer-term intentions. Vice President Kamala Harris largely embraces Mr. Biden’s view of the importance of strategic alliances, though her specific policy views are still coming into focus as she campaigns on a compressed timeline. Former President Donald J. Trump promises a return to his “America First” brand of isolationism, while boasting about his own diplomatic skills.

The world leaders are gathering at the U.N. as multiple global crises are colliding with American politics in a way that could reshape how the United States confronts the world’s most difficult problems.