Yesterday via Twitter.

Those who were around during the Vietnam war have exhausted every possible argument about who did what, and why, and when, and with what justification.

Those who were not around must no doubt have had their fill—though for them and everyone else I highly recommend the new 10-part, 18-hour series on the Vietnam war by the filmmakers Ken Burns and Lynn Novick, which will air on PBS in September (and which I’ve been watching in previews). Not everyone will agree with every part of its emphasis, but everyone will learn from it and, I predict, find it moving. I’ll have more to say about this when it appears.

But for those who weren’t around, here’s a primer on why Donald Trump took yet another step into graceless and obnoxious territory by criticizing Richard Blumenthal, the Democratic senator from Connecticut, for embellishing or lying about his Vietnam-era military record.

The simplest way to think of this is to imagine that favorite of business school courses, a two-by-two matrix. We can classify Vietnam-era figures on two criteria:

1. Did they think anybody in the United States should be going to fight in Vietnam, Yes or No? This is a proxy for the question: Are you for or against the war?

2. Did they think that they personally should go to fight in the  war?

A 2x2 matrix explains it all.

For three of the positions in the matrix, you can imagine moral and character defenses. (The arguments would start when you say which should rank above which others.) Not for the fourth. To take them in order:

1. Committed Warriors Yes, and Yes: This group includes people who thought that somebody should go fight, and were themselves willing to go. Among currently prominent public figures, John McCain would clearly be in this group. So would former Democratic senator and presidential candidate Jim Webb, a wounded and decorated Marine combat veteran, and like McCain an Annapolis graduate. Assuming that Robert Mueller was in favor of the war when he joined the Marine Corps in 1968, he would be another member. Presumably John Kerry, before experience in Vietnam turned him against the U.S. involvement, would also fall into this category.

2.  Reluctant Warriors. No and Yes. These were people who thought no American should have to go and fight—but that if anyone had to go, they should be willing to do so as well. Al Gore is an obvious example. He opposed the war as a college student but nonetheless joined the Army and went to Vietnam, though he was not in combat there. The closest hometown friend of mine who was killed in Vietnam, Christopher Warren Morgens (who was in the same Harvard class as Gore), was in this category, too. As a matter of character, bravery, and patriotism, you could argue that this group is the most admirable of all.

3. Consistent Non-Warriors. No and No. These were people who thought that no Americans should be going off to fight in Vietnam, and that the “no one” should apply to themselves too. This was my own category. As a college student in the late 1960s I was part of the out-of-Vietnam movement, and like many other college students I did my best to keep myself out, too. The brutal fact that it was easier, for opponents of the war, to keep themselves from being involved than to change the whole nation’s policy left this group with its moral ambiguity. Back in the 1970s I wrote a Washington Monthly article called “What Did You Do in the Class War, Daddy?” about the way the college-educated averted their eyes from who was going to war in their stead. But at least they—we—had some logical consistency to their position. We didn’t want to go, and we didn’t want others to go either. Bill Clinton would be in this group.

A variant that straddles several categories would be those who served but in a way calculated to keep them out of Vietnam or combat. The National Guard berths of Dan Quayle and George W. Bush fit this pattern—as, it seems clear, did the Marine Corps reserve service of Richard Blumenthal. (Blumenthal’s varying accounts of his service are another matter.)

4. The Chickenhawks. Yes and No. These are the people who thought: Someone should be over there fighting. But it shouldn’t be me.

Until recently Dick Cheney, he of the pro-war views and the multiple deferments, had been the most politically prominent representative of this category. Now of course it is Donald Trump: star high school athlete, spared from service because of unspecified foot problems, now criticizing other public figures for their war records.

* * *

War is hell, and as the upcoming Burns-Novick documentary shows, the U.S. participation in Vietnam left moral murkyness all around it. Each person weighs the rights and wrongs of what he or she did—he” much more than she in those days, since women were not subject to the draft or exposed to combat. (But many thousands of American women served as nurses and in other functions in Vietnam; the memorial wall lists the names of eight who were killed.)

Through the murk, though, one line shines bright and clear. Even as the United States becomes more and more a “chickenhawk nation”—always at war, but with only a tiny sliver of the country doing the fighting—it’s the line that individual chickenhawks should respect. No one wants to hear them criticizing others for their war decisions, not even via Twitter from the golf course.