It will be the largest gathering of presidents and prime ministers ever at the United Nations. Some of them will assemble here under one roof for the first time in years, and their job will be to wrestle with global crises that they stubbornly disagree on, including climate change, the war in Syria, and a historic exodus of people fleeing conflict and hunger.

But even before they start lecturing one another at the United Nations General Assembly this week, the leaders will come in for a heavy dose of moral flogging by an enormously popular leader known for taking swings at the global elite: Pope Francis.

This is the first time a pope will address such a large gathering of world leaders at the United Nations, according to Ban Ki-moon, the United Nations secretary general. Francis speaks in the General Assembly on Friday morning, before the official start of a global summit meeting where an ambitious set of development goals are to be adopted, including reducing economic inequality and offering good schools for all.

Lifting up the poor is a signature issue for Francis. Yet, he could give world leaders a tongue lashing for ravaging the environment, failing to protect people from mass atrocities and rejecting refugees who show up at their borders.

All are transnational challenges that defy easy answers. In a way, some argue, only a figure of the pope’s stature has the global popularity and moral authority (though no enforcement powers) to make the world powers take heed.

“Governments and their leaders are not easily swayed by moral appeals, even from the pope, but they do respond to their public opinion,” said Louise Fréchette, a former deputy secretary general of the United Nations. She added that the pope’s emphasis on issues like climate change and refugees could put pressure on governments to act.

It is also a boon for the United Nations. “His presence,” Ms. Fréchette said, “underlines the continued centrality of the institution.”

The secretary general certainly seemed to be looking to Francis for support nudging the 193 nations that make up the organization. “We expect that he will send his spiritual guidance to all the member states of the United Nations,” Mr. Ban said, a bit wistfully, at a recent news conference. “I really count on his leadership.”

More than 150 presidents and prime ministers are scheduled to follow Francis at the development summit meeting that starts Friday. Then, on Monday, world leaders are scheduled to begin the annual debate, approaching the podium one by one under the illuminated dome of the General Assembly hall, to advance their agendas and, inevitably, take swipes at their rivals.

The two sessions, which United Nations officials call historic, feature an all-star lineup of autocrats, princes and populists, some of whom rarely show up at the General Assembly.

It will be the first visit by Chancellor Angela Merkel of Germany in seven years and Xi Jinping’s first as president of China. President Vladimir V. Putin of Russia was last here a decade ago. His speech, scheduled for Monday morning, shortly after President Obama’s, will be among the most closely watched.

The General Assembly conclave will produce the first meeting of Mr. Putin and Mr. Obama in nearly a year. The Russian envoy to the United Nations, Vitaly I. Churkin, would say only that he hoped frayed relations between the two countries would improve.

“I don’t think it’s a second Cold War. But it’s very uncomfortable,” he said in a recent interview.

Syria will be front and center in this year’s deliberations. Russia and the United States are planning to host rival sessions, one in the General Assembly and one in the Security Council, on countering terrorist groups in the Middle East. The foreign ministers of all five permanent members of the Council — Britain, China, France, Russia and the United States — will also discuss Syria at a lunch hosted by Mr. Ban.

Those meetings come at a pivotal moment in the Syrian conflict, as Russia sends more men and matériel to aid Mr. Assad’s government, prompting American officials to open up military-to-military talks with the Russians. British and American jets have conducted airstrikes on what they call Islamic State positions. France has announced that it will begin airstrikes soon.

The flurry of diplomacy may simply highlight the failure to prevent “the scourge of war,” as the United Nations Charter set out as its principal goal 70 years ago. Beyond Syria, the conflicts in Darfur, in western Sudan, and in Yemen and Libya continue, often with the involvement of regional and world powers.

(The New York Times)