“As states increasingly perceive the U.N. as ineffective, it becomes a vicious circle: They fund it less, they provide it with less political support,” he warned. “What I worry about is death by a thousand cuts to the U.N. It won’t disappear tomorrow, but unless we radically turn this around, you will see it slowly drift to occupying the margins of global irrelevance.”

Not everyone at the conference shared that view. Amina Mohamed, minister of foreign affairs and international trade in Kenya, defended the United Nations. She said international organizations were “not the problem per se. The problem is us, the member states, because these organizations can only do as much as we allow them to do.”

She listed all the ways in which she said the United Nations had come to the world’s aid: stamping out piracy in the Horn of Africa, stemming the bird flu pandemic and the spread of the Ebola virus. Without multilateralism, she said, “none of the big challenges” faced by the world “would be resolved.”

Challenges certainly face the European Union as it prepares to see one of its 28 member states depart. After last year’s British referendum, European Commission President Jean-Claude Juncker said the union was, “at least in part, in an existential crisis.” In March, he issued a white paper proposing options for the union’s future that involved either deepening or loosening integration.

In a speech last week, Mr. Juncker sounded much more bullish. He said the wind was “back in Europe’s sails,” with growth in the European Union outpacing that of the United States for the last two years, and he pushed for an ever-closer union.

Yet criticism of the union continues, particularly in Britain, even from those who voted to remain in it.

“Of all the democracies in the world, the European Union is the arena where sovereignty is most diluted,” said Paul Mason, a British journalist and filmmaker and a member of Britain’s opposition Labour Party. “Government action is severely constrained by what the European Union will allow you to do.”