“The senator from Kentucky is now working for Vladimir Putin,” fumed John McCain back in March 2017. His target: Rand Paul, who had committed the unforgivable offense of momentarily delaying the latest round of NATO expansion. Montenegro, a tiny country in southeastern Europe that most Americans have never heard of, was about to join the sprawling military alliance — and McCain was determined to see the final ratification ritual proceed with as little debate as possible. So he hurled the time-honored “working for Putin” accusation, and sure enough, Paul quickly withdrew his minor procedural objection. The glorious ascension of Montenegro...