Former defense secretary Robert Gates addresses the Boy Scouts of America’s annual meeting in Nashville. (Mark Zaleski/Associated Press)

Republicans who are climbing aboard the Donald Trump train with no other reason than the “R” after his name (and belief that Hillary Clinton is the devil incarnate) should listen to former CIA director and secretary of defense Robert Gates. He makes clear that in the single most important role — commander in chief — Trump is dangerously, perhaps irretrievably flawed. He had this exchange on “Face the Nation”:

JOHN DICKERSON: You served eight presidents. Are these the kinds of reservations that can be — are these policy position issues or can they be fixed with a good staff?

GATES: They’re policy positions, so they can always change. I have seen presidents do that more than once.

I guess one of the things that makes it challenging for me is that [Trump] seems to think that he has all the answers and that he doesn’t need any advice from staff or anybody else, and that he knows more about these things than anybody else, and doesn’t really feel the need to surround himself with informed advisers. . . .

[Previous presidents] would often make their own independent judgments. They often would act contrary to the advice they were receiving. But, nonetheless, they only acted after they had listened to different points of view and then had the opportunity to make up their mind.

DICKERSON: Is that a fatal flaw?

GATES: I think that you would have to — one would have to show between now and the election, it seems to me, that you were willing to do that, and willing to listen to people, willing to adjust your positions, to give any sense of confidence that — I come at this, all this, from a national security standpoint.

And that’s what I look at. I just — I came to this interview from commissioning 18 brand-new second lieutenants coming out of ROTC. And I think about all those young people in the military and who is going to be in charge of them, who is going to be in position to give them orders.

And so I want to see some evidence that a person can be trusted with the lives of those young people.

That is as sobering a thought as any in the rush to vouch for Trump. Republicans, we would suggest, could not look the men and women of the military — nor their parents — in the eye and say Trump has the judgment, temperament and basic knowledge to lead them. Gates dubbed it “inconceivable” that he would serve if Trump asked him. That same word aptly describes the chance that in six months Trump could learn enough about the world, and at 69 years old become stable, thoughtful and serious enough to make life or death decisions.

Then there is Hillary Clinton. According to Gates, “It was my experience in working with her that she was very tough minded. And, for example, when General McChrystal was asking for 40,000 additional troops in Afghanistan, she was very tough in support of what the general wanted. She also was an advocate for going into Libya. . . . I would suspect that generally speaking she is more hawkish than President Obama.” She also advocated early action in Syria, advice which if followed might have forestalled years of civil war and the present domination of Syria by Russia and Iran. From her comments during the campaign, she has sounded tougher on Iran, for example, in supporting new sanctions.

There is the choice: someone utterly unfit to be entrusted with our troops (and the nation’s national security) and someone likely better than Obama. Maybe a third candidate will come along who stands head and shoulders above them both. If so, for those who don’t simply mouth the platitude that being commander in chief is the most important part of the job, the choice will be much easier. As things stand now, it’s not all that hard. It’s not ideal, but it’s not hard to eliminate Trump as a fit commander in chief.