Washington (CNN) It was a photo-op meant to portray President Donald Trump at his most commanding: Flanked by senior military leaders, Trump was there to plot America's response to a suspected chemical gas attack in Syria.

Instead, his boiling display in the White House Cabinet Room on Monday evening only served to underscore the President's most visible weakness: his ambient rage over Robert Mueller's Russia investigation.

The special counsel, Trump declared, was conducting a "total witch hunt." The FBI raid carried out on his longtime fixer and personal attorney Michael Cohen's office is "frankly, a real disgrace." And the investigation into his campaign's ties to Russia amounts to "an attack on our country in a true sense."

That was just in the first minute.

Rarely has Trump's ever-simmering anger over the Russia probe erupted on camera as it did on Monday. The news that FBI agents had carted away documents and records related to, among other things, the adult film actress Stormy Daniels from Cohen's office prompted the type of emotional diatribe that his allies and advisers have long sought to head off.

That's left his aides and advisers wondering -- and in some cases dreading -- what might come next.

People familiar with Trump's thinking said Monday that the Cohen revelation struck the President harder than seeing other associates caught up in the Mueller swirl. Unlike campaign aides Paul Manafort or Michael Flynn, Cohen has been a longtime fixture of Trump's orbit. He was a regular presence inside the Trump Organization headquarters on the 26th floor of Trump Tower. And he acted as an enforcer of secrets, admitting to paying off at least one woman who alleged she'd carried out a sexual affair with Trump.

He was, in the telling of one person who worked with both Trump and Cohen for years, "the closest person to Trump that I have ever met who is not family."

Trump made the decision on his own to directly -- and bluntly -- address the FBI raid of Cohen's office during his meeting with top military brass and his national security advisers. One White House official said there was no discussion among aides about the President not talking about it at his event.

He wanted to air his grievances, the official said, and react publicly to the news that had enraged him for the past two hours.

"You can see how angry he is," the White House official said.

JUST WATCHED WH Official: Trump watching tv reports of FBI raiding Replay More Videos ... MUST WATCH WH Official: Trump watching tv reports of FBI raiding 01:32

The outburst came late in a day otherwise occupied with calls and meetings focused on the Syria attack. By midafternoon, Trump had spoken for the second time with French President Emmanuel Macron and had conferred with senior aides, including new national security adviser John Bolton, on options for responding.

But the strategy sessions were interrupted when Trump learned the raids were underway. He huddled at the White House with his attorney Ty Cobb and other senior aides to plot a response.

"I heard it like you did," Trump told reporters in the Cabinet Room, though one official said the President had found out earlier in the afternoon that FBI agents had seized documents from Cohen's office and hotel room.

He spent the remainder of the afternoon viewing coverage of the episode on television in the private dining room just off the Oval Office, growing more incensed and planning his response. White House aides seemed caught off guard by the raids, unprepared with a response. Press aides spent the afternoon hunkered down in their offices determining how to react.

Trump, now without a permanent communications director after the departure of Hope Hicks, ended up doing the reacting himself.

"That is really now in a whole new level of unfairness," Trump bemoaned at the start of his meeting with military leaders.

Without being prompted by a reporter's question, Trump launched into his minutes-long tirade as top military brass and national security aides looked on.

Defense Secretary James Mattis sat stone-faced across the table, his hands folded and his eyes cast downward. Bolton, sitting to Trump's left, adjusted his glasses and fiddled with his pen. Vice President Mike Pence, at Trump's right, stared ahead with an unchanging expression of concern.

Other military leaders looked on silently as the commander in chief attacked his attorney general, deputy attorney general and special counsel in a blistering partisan attack that stood out even by Trump's standards.

JUST WATCHED Trump: Syria attack will be met forcefully Replay More Videos ... MUST WATCH Trump: Syria attack will be met forcefully 01:58

Trump diverted from his rampage, briefly, to address the situation in Syria. But he eagerly returned to the Mueller matter as soon as he was peppered with questions from reporters.

"Why don't I just fire Mueller? Well, I think it's a disgrace what's going on. We'll see what happens," he said, making sure to repeat the question for the cameras. "And many people have said, you should fire him."

It's the prospect of a Mueller firing that has some of Trump's advisers nervous. Trump has recently escalated his attacks in private, not just of Mueller but also of Deputy Attorney General Rod Rosenstein, who oversees the Russia investigation at the Justice Department. After the departure of aides like Hicks, there are fewer confidants in the White House to buffer Trump's combative instincts.

A growing number of Trump's allies have advised him that his legal team appears outmatched by the stable of lawyers recruited by Mueller to investigate his campaign's ties to Russia, even as that team begins informally preparing him for an interview with Mueller. And they are concerned about how he will react to the upcoming publicity tour by James Comey, the FBI director he fired last year, who has a book coming out.

Republicans in Washington are eyeing the President's moves carefully.