The annual United Nations General Assembly will unfold this week against a backdrop of crises — from the warming planet to economic uncertainty to flaring conflicts that threaten to further entangle the United States in the volatile Middle East.

Trade wars, migration, energy supplies, climate change and the eradication of poverty underpin the basic themes of the 193-member General Assembly agenda. But the actions of the Trump administration, which has sometimes expressed disdain for international institutions like the United Nations, have created a common denominator.

“All of the major topics that I think people will be talking about in the corridors are related to: What is U.S. policy?” said Jeffrey D. Feltman, a veteran American diplomat and former United Nations under secretary-general for political affairs.

Some leaders are not coming, notably Presidents Xi Jinping of China and Vladimir V. Putin of Russia, as well as Benjamin Netanyahu, the embattled prime minister of Israel. Also not expected is President Nicolás Maduro of Venezuela, regarded by the Trump administration and about 50 other governments as an illegitimate leader.