REUTERS/Mike Segar
Mitt Romney and Barack Obama during a 2012 debate.
"It's a sad commentary that the president's been so ineffective," Romney told conservative radio host Hugh Hewitt.
"I find it troubling and revealing that the world around us is going to hell," he added, "and the president takes no responsibility."
Romney had a long list of criticisms of the Obama administration's actions abroad. In particular, Romney ripped Obama for saying the Islamic State group, also known as ISIS, was geographically "contained" just before last week's terror attack in Paris, France.
"Once again, you have a president not acknowledging that what he told us was wrong," Romney said. "He told us that ISIS was contained and he was wrong. He initially told us that he didn't have a strategy to deal with ISIS, then he tells us that he has a tactical approach. And his approach has not worked."
Romney even offered a prediction that Obama could go down as one of the worst national-security presidents in history.
"I just think that on the issue of foreign policy - and [protecting] America and America's interests, and the interest of freedom secure and strong - the president's been a disaster," he said. "I think he may go down in history as one of the very worst foreign-policy presidents in American history."
He added: "And the consequence of that is loss of life. The consequence of that is the loss of freedom."
Romney, who has otherwise been quiet in recent weeks, stepped up his public profile after the Paris attacks. Stressing that the US needs to step up its efforts against the Islamic State, Romney wrote a Washington Post op-ed on Sunday and gave a "Today" show interview on Monday.
Some members of the Republican establishment, worried about the rise of Donald Trump and Ben Carson, have reportedly discussed trying to draft Romney into the 2016 race. But Romney batted down that speculation again during his "Today" interview.
"I've said it. I'll say it again today, which is: I'm not running," Romney said, shaking his head. "I'm not planning on running."