Let's start from the beginning.

Wednesday afternoon, McCain was working with two Democratic senators to request a Senate vote on allowing Montenegro, a small Balkan nation, into NATO. The Senate vote is one small but important step in Montenegro’s nearly decade-long process to be allowed into the treaty group, which includes much of North America and Europe.

McCain spoke passionately on the Senate floor in favor of letting Montenegro in, most especially because it would serve as a direct rebuke to Russia. Russia, McCain said, “wanted to kill the prime minister and overthrow the government in order to keep Montenegro from becoming part of NATO.”

McCain and his two Democratic colleagues pushed for unanimous consent to approve the treaty ascension. Unanimous consent is a procedure senators can deploy when there's hardly any opposition to their proposal. If no senator openly objects to it, the issue at hand will automatically pass without all 100 senators having to take a vote.

Then Paul walked in, offered his objection and walked away.

McCain was dumbfounded. And visibly angry. Here are his comments in full:

“That is really remarkable. That a senator blocking a treaty that is supported by the overwhelming number, perhaps 98 at least, of his colleagues, would come to the floor and object and walk away. And walk away. The only conclusion you can draw when he walks away is he has no argument to be made. He has no justification for his objection to having a small nation be part of NATO that is under assault from the Russians. So I repeat again: The senator from Kentucky is now working for Vladimir Putin.”

A United States senator is now working for the Russian president. That’s a striking thing to say on the Senate floor ever, let alone at a moment in time when Congress is investigating Russia’s meddling in the U.S. election and the FBI is looking into the president’s ties to Russia.

The next morning, Paul was on MSNBC’s “Morning Joe,” where he was asked by host Willie Geist about McCain’s comment.

Paul said: “You know, I think he makes a really, really strong case for term limits. I think maybe he’s past his prime; I think maybe he’s gotten a little bit unhinged.”

Not that anyone’s innocent in this, but Paul could have answered the question on the policy merits alone and avoided calling an 80-year-old senator “unhinged.” Something like, “You know, the senator is entitled to his opinion. But here’s why I oppose letting Montenegro into NATO” would have worked.

Now Paul is in the position of escalating a name-calling battle. It undercuts Paul’s argument that McCain went too far (and that two people who disagree can have a “rational” discussion on NATO). And from McCain’s perspective, his quote about Paul and Putin being in cahoots overshadows the reason he said it.

(McCain’s office explained in a statement to The Fix on Thursday: “Senator McCain believes that the person who benefits the most from Congress’s failure to ratify Montenegro’s ascension to NATO is Vladimir Putin.")

Paul did go on to explain on MSNBC why he objected to allowing Montenegro into NATO. It would make the United States more vulnerable to situations where it might have to use force, said Paul. He and McCain exist on opposite ends of the spectrum of foreign policy views within their party. Where McCain believes military force abroad can be a tool for good, Paul is very, very skeptical of its power and any circumstance that might lead the United States to have get involved in other nations’ affairs.

We get it. When two people disagree about important things, passions can run high, and sometimes you say stuff you regret.

But this Paul-McCain back-and-forth doesn’t feel like an isolated personality-driven quarrel.