'Hardball' host Chris Matthews asked presumptive Republican presidential nominee Sen. John McCain (R-AZ) at a town hall-style meeting what the 'tipping point' would be for the US to use military force against Iran.

Stressing the need for diplomacy, McCain ultimately replied, "It would have to be made clear to the overwhelming majority of Americans that there was a clear and present danger.

"If we ever have to go to the American people and send these young people in harm's way," said the Arizona senator, "we're gonna to have to make an even more convincing argument that it was necessary to do so because of our failure to find weapons of mass destruction [in Iraq]."

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McCain did repeatedly express what he felt was Iran's significant role behind events in the Iraq theater of war. "I don't think there's any doubt about Iranian ambitions in the region," he said.

In a lighter moment, McCain also quipped to an audience member, "We now have a pro-American president of France, which shows if you live long enough, anything can happen in this world."

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