In Prague, the two leaders expressed a commitment to their strong alliance, but there are underlying tensions stemming from the U.S. plans for a missile defense system. Medvedev warned once again that the Russians could pull out of the treaty if they believed the plan undermined the new foundation of arms control.
Obama disputed that notion to Stephanopoulos and said the Russians were not saying they would withdraw from the treaty over the missile defense system.
"We were firm, and are now absolutely confident that this in no way impedes our ability to move forward on the missile defense program, that's designed not to target Russia," he said.
Obama said that Virginia Republican Gov. Bob McDonnell's proclamation designating April "Confederate History Month" without a mention of slavery was "an unacceptable omission" but one he has now acknowledged.
"I'm a big history buff. And I think that understanding the history of the Confederacy and understanding the history of the Civil War is something that every American and every young American should be a part of," the president said. "Now, I don't think you can understand the Confederacy and the Civil War unless you understand slavery. And so, I think that was an unacceptable omission. I think the governor's now acknowledged that."
Both Obama and Medvedev pledged Thursday to crack down on Iran with sanctions if it continues to advance its uranium enrichment program and pursue nuclear weapons.
Obama told Stephanopoulos that he and Medvedev are in agreement that there needs to be "sanctions that will change the behavior of Iran.
"I think it's an enormous shift and a signal that Russia, like the United States, recognizes that unless we can get all countries to start abiding by certain rules of the road, and right now, our biggest concerns are obviously Iran and North Korea," he said.
But Obama cautioned that given Iran's unpredictable behavior in the past, it's difficult to guarantee success.
"If the question is do we have a guarantee as to the sanctions we are able to institute at this stage are automatically going to change Iranian behavior, of course we don't," he said. "The history of the Iranian regime, like the North Korean regime is that, you know, you apply international pressure on these countries, sometimes they choose to change behavior, sometimes they don't."
Asked why he had confidence that the continued pressure and threat of sanctions on Iran could work this time, Obama said there has never been "the degree of international unity that you've seen in this effort," but he admitted that Iran has not showed yet "the kinds of signs that would satisfy me."
"And that's why I think we've got to keep on pursuing all options," he said. "And at this point, the most important option in front of us is-- is strong and vigorous sanctions."
Critics say both the Obama Administration's new nuclear policy and the U.S.-Russia arms agreement send the wrong message to Iran, even as Obama administration officials argue that it puts more pressure on Iran to stop its pursuit of nuclear weapons.
Obama brushed off Iranian President Ahmadinejad's recent comment that the American president was an "inexperienced amateur."