Biden faces an uphill climb as he tries to reassure allies at the U.N. General Assembly.

At the beginning of the summer, President Biden’s aides had every reason to believe the opening of the U.N. General Assembly, the annual diplomatic Super Bowl of self-congratulatory speeches and closed-door meetings, would be a pretty glorious moment. After all, he wasn’t Donald J. Trump.

Covid-19 appeared to be receding. The president had a plan to get all American troops out of Afghanistan — and thought the Afghan government could hold on. A deal to build nuclear-powered submarines together seemed like a great way to put new military pressure on China, even if the French were likely to get out of joint when a major defense contract was overtaken.

Now, as Mr. Biden prepares to speak for the first time as president from the United Nations, nothing looks quite that easy.

His familiar refrain that the world must choose between democracy and autocracy looks different now that the Taliban are once again in control of Kabul, reversing many of the democratic gains of the past 20 years. Covid is resurging in much of the world. And the French just recalled their ambassador in outrage — not just over losing a $60 billion-plus submarine contract, but because it was made clear they are not in the inner circle of allies.

Mr. Biden has a chance to reverse all that. But it will be an uphill climb, with allies in the room — not just France — questioning how much in America has changed since Mr. Trump left office. Mr. Biden will argue that a lot has, and insist he is not spinning toward a Cold War with China.

“He will make absolutely clear he is not looking to pursue a future — a new Cold War with any country in the world,’’ Jen Psaki, his press secretary, told reporters on Monday. “And in between a one-day blitz through New York, he is trying to reach President Emmanuel Macron of France, to reaffirm our commitment to working with one of our oldest and closely partners,’’ she added.

The allies recognize, of course, the differences between Mr. Biden and Mr. Trump. But in conversations over the past two weeks, they say they have new concerns about the United States.

They worry about whether Mr. Biden really has their back, after the French foreign minister compared the submarine deal with Australia to a “knife in the back.” When they hear about booster shots in the United States, they usually wonder what that might do to global supplies of vaccine. And when they look at how the U.S. handles the Australia deal, they wonder whether American national interest has now eclipsed the role of global leader.

Mr. Biden traveled to New York on Monday afternoon, and met with the Secretary General of the United Nations, António Guterres. On Tuesday he is only scheduled to meet one-on-one with a single ally: Prime Minister Scott Morrison of Australia, whose decision to enter the submarine deal — and work on cyber and space technology — casts his country far more firmly in the American camp.

On Wednesday, from the White House, Mr. Biden will convene a group of nations on Covid-19 responses, announcing plans to speed up distribution of vaccine doses around the world.

But there will be competition: Xi Jinping of China, who will address the General Assembly on video from Beijing, making the competing case.