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The five permanent members of the Security Council — the United States, Russia, China, France and Britain - along with Germany and the European Union, will send representatives to meet with Iranian representatives on Oct. 1 to discuss nuclear weapons, though Tehran has indicated it will talk about nuclear proliferation broadly without discussion of its own nuclear program.

And next spring, Mr. Obama will host a conference in Washington aimed at locking down all loose nuclear materials around the world in four years.

In a broader sense, Mr. Obama’s decision to chair the Security Council was a sign of respect for the international body, aimed at strengthening its ability to level effective sanctions against countries that develop nuclear weapons in violation of the NPT. Relations between the United Nations and the George W. Bush administration were strained.

The chair of the Security Council rotates monthly on an alphabetical basis. The United States and most other countries usually send a high-level government representative to chair the meeting.