At the 81st United Nations General Assembly in New York, Donald Trump arrives bearing the weight of a nation reassessing its place in a straining world order. His central argument — that Iran must not be permitted to develop nuclear weapons — is both a policy position and a signal of how the United States intends to exercise its influence in what officials call a consequential year. The week of bilateral meetings ahead will test whether American diplomacy can build bridges or whether the fractures of a changing global system run too deep for any single gathering to mend.