In a speech that also highlighted his early opposition to the Iraq war, Obama said he doesn't want the United States to disarm unilaterally, but instead to work with other nations to phase out nuclear weapons and control nuclear materiel.
"America seeks a world in which there are no nuclear weapons," Obama said.
"The best way to keep America safe is not to threaten terrorists with nuclear weapons - it's to keep nuclear weapons and nuclear materiels away from terrorists," the Illinois senator said. Aides said the process Obama envisions would take many years, not just a single presidency.
Republicans criticized Obama's stance as dangerous to national security.
"In a world with terrorists trying to acquire nuclear technology and with Iran and Syria threatening their neighbors, it is difficult to comprehend that a major presidential candidate wants to eliminate our nuclear arsenal," said Republican National Committee spokesman Danny Diaz. "This is yet another example of Senator Obama playing to the fringe elements of his party and failing to understand the threat America faces."
Obama delivered the foreign policy address on the fifth anniversary of his speech at an anti-war rally where he announced his opposition to invading Iraq. He predicted then that the United States would get bogged down in an unending war that would inflame world anger.
Speaking at DePaul University, Obama pointed out that the campus was filled with students for whom the Iraq war has been a constant for four years.
Obama was a state legislator in Illinois when Congress voted in October 2002 to give President Bush the authority to use military force to oust Iraqi leader Saddam Hussein. In his speech, Obama criticized Bush, the media and Congress, arguing they failed the nation in the rush to war.
"Let's be clear: Without that vote, there would be no war," Obama said in his speech.
Obama took a swipe at his Democratic rivals who were in the Senate and voted for the war -, , and - but never mentioned them by name.
"Some seek to rewrite history. They argue that they weren't really voting for war, they were voting for inspectors, or for diplomacy. But the Congress, the administration, the media and the American people all understood what we were debating in the fall of 2002," Obama said. "And we need to ask those who voted for the war: How can you give the president a blank check and then act surprised when he cashes it?"
Obama cites his early opposition to war as evidence that he has the judgment to be president despite arriving in Washington less than three years ago.
Voters "should ask themselves: Who got the single most important foreign policy decision since the end of the Cold War right and who got it wrong?" Obama said.
Obama plans to focus on that question all week to counter concerns that other candidates, particularly Clinton, have far more experience in a dangerous era for the United States. In a dig at this rivals, Obama said Congress had failed American citizens on Iraq, despite a law passed after Vietnam that was meant to serve as a check on the president's ability to take the country to war.
"No law can force a Congress to stand up to the president. No law can make senators read the intelligence that showed the president was overstating the case for war. No law can give Congress a backbone," he said.
He promised an aggressive new approach to international affairs. As president, Obama said, he would personally conduct negotiations with other nations, including hostile countries.
He promised an annual "state of the world" speech to assess the country's foreign policy concerns.
He proposed giving the director of national intelligence a fixed term of office, so that he could not be replaced by the president for political reasons.