Though former Defense Secretary Jim Mattis only mentions his last boss, President Trump, in the opening pages of his new book, the Washington establishment seems certain the tome on leadership is directed at him, with one observer calling it “mainly a 100,000-word subtweet.”

Mattis says he won’t discuss a sitting president, though it seems clear he disdains Trump’s leadership style. But he is scathing about a former president in Call Sign Chaos: Learning to Lead, which is released today, and he doesn’t hesitate to name him: Barack Obama.

The former general's opinion of Obama, for whom Mattis was head of U.S. Central Command from 2010 until 2013, might be summed up by a single entry in the book’s index: “Obama, Barack, strategic thinking lacking.”

That entry points readers to a couple of the book’s most thoughtful passages on the necessity of allies and the dangers of alienating them. Those who have heard Mattis in his publicity campaign emphasize these points assume he’s speaking indirectly of Trump: for example, his disparagement of NATO. But Mattis makes clear in Call Sign Chaos that he believes Obama bungled Middle East policy and, as a result, sent American allies around the world a dangerous message.

Mattis summarizes his time commanding CENTCOM, overseeing military operations in the Middle East and Central Asia until Obama fired him, in harsh terms: “It was to be a time when I would witness duty and deceit, courage and cowardice, and, ultimately, strategic frustration.” The general was in charge of two wars, in Iraq and Afghanistan, though one ended on his watch — or so the president said.

“You know I say what I mean, and I mean what I say,” Obama said in fall 2012, as Mattis notes. “I said I’d end the war in Iraq. I ended it.”

Mattis follows this up with an even more succinct statement: “Rhetoric doesn’t end conflicts.”

He details in the book how Obama and Vice President Joe Biden, who was in charge of Iraq policy, were “ignoring reality” in the country and made a political decision to withdraw troops, a choice that allowed the return of al Qaeda in a new and more ambitious guise, the Islamic State.

[ Related: Jim Mattis book already in stores and discounted by 43%]

“In Washington, the debate swirled throughout 2011 about how many, if any, U.S. troops should remain in Iraq,” Mattis writes, after the American-led coalition had established “a fragile stability” in the country after President George W. Bush sent a surge of troops in 2007. “Central Command, the Chairman of the Joint Chiefs, and the new Defense Secretary, Leon Panetta, who had replaced Bob Gates, continued to recommend to the White House retaining a residual force, as did Secretary of State Hillary Clinton,” Mattis writes. But they were “talking to the wind.”

The White House “dealt with Iraq as a ‘one-off,’ as if the pullout of our troops there would have no regional implications, reinforcing our allies’ fears that we were abandoning them. I argued strongly that any vacuum left in our wake would be filled by Sunni terrorists and Iran.”

Mattis believes he was vindicated by events. Obama declared the war over, but “Iraq slipped back into escalating violence. It was like watching a car wreck in slow motion,” Mattis says. “All of this was predicted — and preventable.”

Obama made “catastrophic decisions” in Iraq, Mattis concludes. And he did so because he ignored the advice coming from multiple military and civilian advisers, thinking he knew better than all of them.

“At the top, then as now, there was an aura of omniscience. The assessments of the intelligence community, our diplomats, and our military had been excluded from the decision-making circle,” Mattis writes.

Some readers might take his conclusion out of context and think he’s assessing the sitting president. But Mattis is clear that he’s talking about Obama.

“It’s frustrating to listen to any leader blame his predecessor, especially a political leader regarding a situation that he knew existed when he ran for office,” Mattis says, referring to Obama's disdain for Bush. “Wise leadership requires collaboration; otherwise it will lead to failure.”

Mattis also faults Obama on Syria. He says he’d never seen refugees as “traumatized” as the ones he saw in Jordan who had fled Bashar Assad’s brutal dictatorship.

“CENTCOM had kept a keen eye on Assad’s stockpile of chemical weapons, and we were picking up indications that he was preparing to use them against his own people,” Mattis writes. To stop this from happening, in August 2012, he says, “President Obama issued a firm warning. ‘That’s a red line for us,’ he said. ‘There would be enormous consequences if we start seeing movement on the chemical weapons front or the use of chemical weapons.’”

“A short time later, Assad did employ chemical weapons, killing hundreds of civilians,” Mattis continues. “At CENTCOM, I assumed we would be the ones to provide the President’s ‘enormous consequence.’ We prepared options to hold Assad harshly accountable, with NATO and Arab allies in support, from single strikes to more extensive operations, depending on the President’s judgment.”

But Obama declined to do anything. “This was a shot not heard around the world,” Mattis writes. And the effects were felt far beyond Syria and even the Middle East.

“Old friends in NATO and in the Pacific registered dismay and incredulity that America’s reputation had been seriously weakened as a credible security partner,” Mattis writes. “Within thirty-six hours, I received a phone call from a friendly Pacific-nation diplomat. ‘Well, Jim,’ he said, ‘I guess we’re on our own with China.'”

Mattis concludes, “Over the next several years, Syria totally disintegrated into hell on earth. The consequences included an accelerated refugee flow that changed the political culture of Europe, punctuated by repeated terrorist attacks. And America today lives with the consequences of emboldened adversaries and shaken allies.”

The Marine general acknowledges “I’m known for blunt speaking,” and it seems it was his insistent frankness that got him fired as CENTCOM commander. He was relieved of command without even an official phone call.

“In December 2012, I received an unauthorized phone call telling me that in an hour, the Pentagon would be announcing my relief,” he writes. “I was leaving a region aflame and in disarray. The lack of an integrated regional strategy had left us adrift, and our friends confused. We were offering no leadership or direction. I left my post deeply disturbed that we had shaken our friends’ confidence and created vacuums that our adversaries would exploit.”

He concludes his discussion of his CENTCOM years with another of his impassioned statements on the crucial value of allies — and this one is referenced in the index under “Obama, Barack, strategic thinking lacking.”

“Under our form of government, the President is our Commander in Chief and must be the sentinel for our nation’s future generations. This calls for a strategy both embraced by the American people and inclusive of our allies,” he writes. “We’ve fought wars that we should have avoided, and half-heartedly engaged in wars that needed to be won. But we can recover our strategic footing, if we don’t squander opportunities to strengthen the international order that is in the best interest of all nations seeking peace and stability on the world stage.”

He has one last parting shot for his old boss Obama: “Acting strategically requires that political leaders make clear what they will stand for and what they will not stand for. We must mean what we say, to both allies and foes: no more false threats or failing to live up to our word.”