UNITED NATIONS -- Even as world leaders step up to the podium at the U.N. General Assembly on Wednesday, the lion's share of attention will be down the hall, where President Trump will be chairing the Security Council. It will be his first experience in leading a session of the U.N.'s most powerful body, where the U.S. currently holds the rotating presidency - a perch it's using to double down on its criticism of Iran.

While Wednesday's council meeting will be on the issue of nonproliferation of nuclear, chemical and biological weapons, Mr. Trump has left little doubt that it'll be another chance to target Tehran.

On Tuesday, during an unabashedly "America First" speech, he said Iranian leaders "sow chaos, death and destruction" and "spread mayhem across the Middle East and far beyond." His national security adviser, John Bolton, warned there would be "hell to pay" if Tehran crossed the U.S., its allies or their partners.

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Iranian President Hassan Rouhani responded by accusing the Trump administration of violating the rules of international law and "state obligations" by withdrawing from the 2015 nuclear deal that Iran signed with the U.S. and five other major powers.

Rouhani is almost certain not to attend the Security Council meeting that will test Mr. Trump's ability to maintain diplomatic decorum and interact with representatives of rival nations.

Mr. Trump's Tuesday speech drew headshakes and even mocking laughter from his audience of fellow world leaders.

"The U.S. will not tell you how to live and work or worship," he said as he unapologetically promoted his "America First" agenda. "We only ask that you honor our sovereignty in return."

Speaking in glowing terms, Mr. Trump approached his address to the world body as something of an annual report to the world on his country's progress since his inauguration. He showcased strong economic numbers, declared that the U.S. military is "more powerful than it has ever been before" and crowed that in "less than two years, my administration has accomplished more than almost any administration in the history of our country."

Just sentences into the president's remarks, the audience began to chuckle and some leaders broke into outright laughter, suggesting the one-time reality television star's puffery is as familiar abroad as it is at home. Mr. Trump appeared briefly flustered, then smiled and said it was not the reaction he expected "but that's all right."

Later, the president scolded Germany for relying on Russian fossil fuels.

"Germany will become totally dependent on Russian energy if it does not immediately change course," Mr. Trump said. "Here in the Western hemisphere we are committed to maintaining our independence from the encroachment of expansionist foreign powers."

Even as he took a swipe at them, German diplomats were seen laughing in the audience.

Later he brushed off the episode, telling reporters, "Oh it was great. Well, that was meant to get some laughter, so it was great."

The leaders' spontaneous response to Mr. Trump's address only reinforced the American president's isolation among allies and foes alike, as his nationalistic policies have created rifts with erstwhile partners and cast doubt in some circles about the reliability of American commitments around the world.

The Security Council is populated by five permanent members - the U.S., China, Russia, Britain and France - and 10 other member states, who occupy a council seat for two-year terms. Iran isn't among them.

Business will continue Wednesday at the General Assembly, where for a second day, 193 U.N. members take turns to speak out on pressing world issues and their national priorities in world affairs.

Among those tentatively scheduled to speak are the leaders of Panama, Iraq, Colombia, Afghanistan and Cuba.

One-hundred-thirty-three 133 world leaders have signed up to attend this year's assembly session, which ends Oct. 1, a significant increase from the 114 leaders last year.

However, America's go-it-alone attitude and growing divisions among key world powers risk eroding the U.N.'s ability to bring positive change in global affairs and end conflicts in Syria, Yemen, and elsewhere.