What Republican presidential candidate Donald Trump told supporters in Greenville, N.C., on Sept. 6, 2016, is raising questions about his "fool-proof" plan to defeat Islamic State militants. (Sarah Parnass/The Washington Post)

Donald Trump has long assured us that he has a very real — and very good — plan to defeat the Islamic State. But he couldn't share the details of it.

He has been talking about this secret plan, in fact, for more than a year. He has said it was "foolproof" and "absolute" in its effectiveness. But he had his reasons not to talk about it.

"I don’t want the enemy to know what I’m doing," Trump told Fox News in May 2015, the month before launching his presidential campaign. "Unfortunately, I’ll probably have to tell at some point, but there is a method of defeating them quickly and effectively and having total victory.”

He added: “All I can tell you it is a foolproof way of winning, and I’m not talking about what some people would say, but it is a foolproof way of winning the war with ISIS." ISIS is an alternative acronym for the Islamic State.

Then, in a June 2015 meeting with the Des Moines Register editorial board, he offered a different reason for keeping the plan under wraps — that his opponents would steal it. "The problem with politics is if I tell you right now, everyone else is going to say, 'Wow, what a great idea.' You're going to have 10 candidates go and use it, and they're going to forget where it came from, which is me. But no, I have an absolute way of defeating ISIS."

He similarly cited the need for keeping his plan secret in an April foreign policy speech: "I have a simple message for [ISIS]: Their days are numbered. I won’t tell them where and I won’t tell them how. ... We must as a nation be more unpredictable. We are totally predictable. We tell everything. We’re sending troops; we tell them. We’re sending something else; we have a news conference. We have to be unpredictable. And we have to be unpredictable starting now. But they’re going to be gone. ISIS will be gone if I’m elected president. And they’ll be gone quickly."

Well, now we know what Trump's "foolproof" and "absolute" plan for defeating ISIS is — to ask the generals to come up with a plan, quickly. That's what he said he would do during a speech Tuesday. "They'll have 30 days to submit to the Oval Office a plan for soundly and quickly defeating ISIS," he told supporters in Greenville, N.C.

These are the generals, mind you, who Trump has said don't understand ISIS like he does. "I know more about ISIS than the generals do," he said in a rambling Iowa speech in November. "Believe me."

The charitable conclusion that could be drawn here is that Trump has decided to abandon his secret plan — or at least set it aside — in favor of letting the generals come up with their own plan. Perhaps he decided the generals actually do know more about the Islamic State than he does. But that doesn't really make sense if Trump's plan was anywhere near as great as he said.

The less charitable conclusion is that Trump never really had a detailed plan.

Trump says many bold things, and it can be easy to dismiss his past comments about his apparently nonexistent plan to defeat ISIS as just another example of his bluster. But on the list of things on which Trump have over-promised and under-delivered, this surely ranks toward the top. He promised he had a "foolproof" way of ending a foreign policy challenge of massive consequence, and now he's punting to the generals.

Unfortunately for Trump, his latest comments play into what Hillary Clinton has been alleging all along.

"The secret, of course, is he has no idea what he’d do to stop ISIS," Clinton said in June.

In just a few hours, the two of them will be discussing weighty matters of foreign policy during an NBC commander-in-chief forum on Wednesday night. This one will probably come up.