When it comes to make-believe foreign policy, usually nothing beats the annual meetings of the UN General Assembly. Each September, member nations of the organization that President Obama recently decried as "paralyzed" each get to perform, sometimes antically, at the podium. And absolutely nothing gets done.

That's especially the view in Washington, which more often than not sees the big green building on the East River as a giant, musty encumbrance. But suddenly the United Nations has become freshly relevant, more so than it has been in years, certainly for all of Obama's first term. And new UN Ambassador Samantha Power, who is largely untested as a diplomat, finds herself in a very hot spotlight, one that might even make her predecessor, new National Security Advisor Susan Rice, a touch regretful that she departed New York so soon.

Indeed, in coming months the actions of the UN Security Council could determine Obama's major foreign-policy legacy, even more so than his takedown of Osama bin Laden, on the long-festering issues of Iran's nuclear program and Syria's civil war.

In both cases a legal dependency on the UN Charter and previous Security Council resolutions will be crucial to success. It already seems clear that it was, more than anything, the sharp bite of UN-approved sanctions on Iran that led to the surprise election of moderate President Hassan Rouhani, who has practically tripped over himself offering to negotiate and whose much-anticipated speech Tuesday is expected to give clues as to his flexibility. It is also clear that previous UN Security Council resolutions dating back to 2006 and demanding that Tehran suspend uranium enrichment will, more than anything else, put Rouhani's sincerity and internationalism to the test. On Syria -- an issue on which Obama has looked consistently weak for two years -- it is also a UN Security Council resolution that will enforce the deus-ex-machina deal that Moscow and Washington suddenly already agreed upon to dismantle Bashar al–Assad's chemical weapons.