During Thursday's White House press briefing, press secretary Sarah Sanders said Trump was just using "common courtesy."
I'll leave that debate to others. What I am most interested is why Trump saluted at all. And I can think of two broad theories to answer that question.
Trump made very clear in the run-up to the Singapore summit that he wasn't doing much in the way of preparation because, well, he didn't think he needed to.
But simply because someone told Trump -- or told someone to tell Trump -- what saluting any North Korean military member might mean doesn't mean that Trump listened or remembered (or cared).
It's uniquely possible that Trump saw a military man saluting him and responded, out of habit, in kind. And that he neither thought about it before it happened or after it was over.
There's little question that Trump went into the summit with Kim thinking that you catch more flies with honey than with vinegar.
It was clear in the final days before the summit, however, that Trump was in flattery mode. He expressed optimism that the summit would work out and that he and Kim would get along well.
When Trump strode out to shake Kim's hand, he did so warmly, putting his left hand on Kim's elbow in a welcoming gesture. From that point until the moment Trump left Singapore, everything he did and said was designed to make clear that he respected Kim and wanted to get a denuclearization deal done.
Saluting a North Korean general who saluted him is totally in keeping with sending a message of respect to Kim -- and keeping alive, in Trump's mind, the possibility of a deal.
Which is it? Did Trump blunder into a quasi-international incident? Or did he purposely salute as a part of a broader strategy aimed at putting the North Koreans in the best possible mindset to make a deal?
As always with Trump's motivations, it's anybody's guess.