Obama at a campaign event in Milwaukee this past weekend. (Barack Obama/Flickr)

"Make no mistake: a nuclear-armed Iran is not a challenge that can be contained," Obama said. "It would threaten the elimination of Israel, the security of Gulf nations, and the stability of the global economy. It risks triggering a nuclear-arms race in the region, and the unraveling of the non-proliferation treaty. That's why a coalition of countries is holding the Iranian government accountable. And that's why the United States will do what we must to prevent Iran from obtaining a nuclear weapon." 

Republican presidential candidate Mitt Romney and Israeli Prime Minister Benjamin Netanyahu have pushed Obama to be more confrontational with Iran. The president's annual speech at the United Nations General Assembly provided him with a big stage to do so.

In his speech, Obama also paid tribute to Ambassador Chris Stevens and decried the video that sparked protests in the Middle East. He said the attacks on U.S. diplomatic outposts in Libya and Egypt were not just an attack on America, but an attack on U.N. ideals.

"There are no words that excuse the killing of innocents," Obama said. "There is no video that justifies an attack on an embassy. There is no slander that provides an excuse for people to burn a restaurant in Lebanon, or destroy a school in Tunis, or cause death and destruction in Pakistan."

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