Tonight, Sen. John McCain (R-AZ) appeared on "The O'Reilly Factor" to discuss his feud with Sen. Rand Paul (R-KY), which started as a difference of opinion on how to address the threat posed to the the U.S. by overseas terrorists, but has since developed into something more personal after Paul's recent unfounded accusation that McCain met with ISIS representatives in Syria.

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In an interview with The Daily Beast earlier this week, Paul said, “Here’s the problem. He [McCain] did meet with ISIS, and had his picture taken, and didn’t know it was happening at the time.”

McCain said he understands that fighting terrorism overseas is a complicated and dangerous situation, but he doesn't see why Paul would repeat unsubstantiated - and untrue - rumors.

In fact, The Washington Post Fact Checker gave Paul the maximum of four Pinoccios for his false claim.

"I don't understand why he would say things that are patently false about me," McCain said. "Frankly, that's a bit disturbing, because that's not how I'm used to working in the United States Senate."

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He pointed out that he was not meeting with representatives from ISIS, but members of the Free Syrian Army, who are fighting against both ISIS and the regime of Syrian President Bashar al-Assad.

“We’re sending them into an environment where Bashar al-Assad can kill them from the air," McCain said. "That’s not fair to them. That’s just wrong.”

“You’ve gotta take out Bashar al-Assad’s airforce.”

McCain also made comments on a moving speech Ukrainian President Petro Poroshenko and called the lack of response and aid for Ukraine “shameful.”

Watch the clip above.

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