But as the anniversary approached and Trump fired his national security adviser, Paul tweeted on Sept. 10: "The threat of war around the world has been greatly diminished, with John Bolton out of the White House." The next day, he reposted criticism from Cheney's opponents and added: "Why do some neocons continue to advocate for endless wars? I stand with @realdonaldtrump on ending wars."

Cheney shot back: "I stand with @realDonaldTrump and our men and women in uniform who will never surrender to terrorists, unlike @RandPaul, who seems to have forgotten that today is 9/11."

For the record, both Cheney and Paul have opposed Trump at various times — something they would have in common if they became colleagues in the Senate. While Cheney has ascended to the third spot in House Republican leadership, she has the option of running for an open Senate seat in Wyoming in 2020.

Paul, meanwhile, is not up for reelection next year, but he does not tend to walk away from fights.

This one is tinged by political legacies. Paul, a libertarian, is the son of former Texas Rep. Ron Paul. Cheney's interventionist approach echoes that of her father, Dick Cheney, who was President George W. Bush's vice president on 9/11.