Donald Trump appears at a campaign event in Toledo, Ohio, U.S., October 27, 2016. REUTERS/Carlo Allegri Donald Trump was quick to declare a military operation in Iraq a "disaster" before it had even really begun. And he has often railed against the US military announcing its operations ahead of time, a move he contends robs the US of the "element of surprise."

But respected military experts are not quite in agreement with his analysis. In fact, some argue the Republican presidential nominee's comments betray his lack of knowledge about military strategy.

Gen. John Allen, a retired four-star Marine general and former special envoy to the US-led anti-ISIS coalition, condemned Trump on a Thursday conference call hosted by Hillary Clinton's campaign.

Allen said that while he "didn't think [he'd] ever be involved in a presidential campaign," he has "agonized" over the rhetoric he's heard from Trump recently.

"While [Trump] appears not to understand basic military activities, it's not clear to me that he's interested in learning them either because there's a number of experts who I think … would advise him otherwise, but he seems to be confident in that knowledge," Allen said.

Trump's most recent bashing of the US military centered around the operation to retake Mosul, a major city in Iraq, from the terrorist group ISIS.

On Sunday, Trump tweeted: "The attack on Mosul is turning out to be a total disaster. We gave them months of notice. U.S. is looking so dumb. VOTE TRUMP and WIN AGAIN!"

And, in an interview on Thursday, Trump said he could teach military experts "a couple of things" about strategy. He again repeated his refrain about the "element of surprise."

"He was trying to play up surprise, but he doesn't understand the reasons at all why we've been forecasting this, which is to get as many civilians out as possible and to push for as many defections from ISIS as possible," said Clint Watts, a fellow at the Foreign Policy Research Institute and former Army officer.

Watts explained the strategy of encouraging any ISIS supporters within Mosul to defect.

"[ISIS] just lost [the symbolic Syrian town] Dabiq, so if you take Dabiq first, then maybe you get a lot of defections and people running for the hills," he said. "That's why they kept saying 'We're starting now.' When you have the Iraqi army and a lot of these units that aren't the American army, you want to thin the herd as much as possible before you send them in."

It was also no secret that the Iraqi army was moving in on Mosul — it's ISIS' last major stronghold in Iraq, and Iraqi forces have been retaking surrounding towns as they close in on Mosul.

"There is no surprise when you're taking Mosul after two years," Watts said. "[ISIS] can see the Iraqi army surrounding them on the periphery."

Watts blamed bad advisers.

"Really this comes down to his advisers," he said. "He's got, in my opinion, horrible counterterrorism and national security advisers."

Republicans have noted that Trump's list of national security advisers lacks heft, and Gen. Michael Flynn, one of the most well-known names on Trump's national security team, has become a controversial figure.

Several other prominent military leaders have also condemned Trump's comments.

Jeff McCausland, a retired Army colonel and former dean at the Army War College, told The New York Times that Trump's bashing of the Mosul offensive shows he "doesn't know a damn thing about military strategy."

Robert Gates, a former defense secretary who served under both George W. Bush and President Barack Obama, told Politico that he guesses "most folks on active duty don't take seriously what he has to say."

And George Little, a former spokesman for the CIA and Defense Department, called Trump's comments "shameful."

That said, some have defended Trump's rhetoric.

Michael Pregent, an adjunct fellow at the Hudson Institute and former US Army intelligence officer in Iraq, conceded that the New York businessman might have a point about Mosul. Pregent returned this week from touring the front lines with peshmerga fighters in Iraq.

"You don't want to reveal too much," Pregent said. "And I think the [Obama] administration reveals way too much. We know when 500 guys are going to an airbase south of Mosul. We know numbers of US advisers. We announce when they're going. I think those are the types of things you're not supposed to do."

Pregent did admit, though, that it's necessary to prepare civilians inside the city for the coming fight.

"Donald had called out tipping our hand, telling the enemy what we're going to do, but part of that strategy is to prepare the civilian population," he said.

Pregent also warned that the worst of the Mosul battle — bloody urban combat inside the urban center — is yet to come.

"The way I look at it right now, it's going to be easy to claim that it's going well when you're basically moving into unoccupied towns," he said. And even in those towns, forces "still have to deal with ISIS snipers, ISIS [improvised explosive devices], and ISIS pockets of resistance."

The bulk of the Mosul operation is still to come; forces have not yet begun to liberate the city itself and are currently still focused on the outskirts.