During 4½ years as secretary of defense under Presidents George W. Bush and Barack Obama, Robert M. Gates was widely lauded as a shrewd national security mandarin who had seen it all, done it all and most important, could stay above it all in the partisan wars of Washington.

So the snarky put-downs and petulant asides in his impassioned, if somewhat contradictory, memoir, “Duty: Memoirs of a Secretary at War,” come as something of a surprise.

Behind his mask of calm civility, Gates nursed grudges, tallied up slights and jotted down the caustic ripostes that he never delivered in public. The carping and zingers already have generated headlines. No crime there, but since he also bemoans the backbiting and bickering in politics, he lowers his pedestal considerably to do so.

Gates is the embodiment of a Washington trope: the independent outsider as consummate insider. He has worked for eight presidents since Lyndon Johnson while proclaiming his loathing for it all. His most recent book covers a particularly turbulent period, from December 2006 to June 2011, when the Pentagon was fighting – and in danger of losing – the wars in Iraq and Afghanistan. Running two major wars and the world’s biggest military was tough enough. Doing it in the last two years of Bush’s second term, and the first two years of Obama’s first, could cause whiplash. So can Gates’ book, which ranges from generous and gracious to churlish and self-righteous, sometimes on the same page. But it is compelling.

Gates is a foreign policy realist, so he views the world through relatively non-ideological eyes. Put another way, he adjusts course and counsel as the winds dictate. It doesn’t make his observations of White House decision-making, and his indictment of Washington’s dysfunction, any less trenchant.

Bush and his aides squandered the initial military victories in both wars by mistakes and short-sighted policies, he admits. Although Gates doesn’t mention the Abu Ghraib prison scandal or other military abuses of that era, he implicitly criticizes his predecessor, Donald Rumsfeld, by forcing a reluctant Pentagon to finally build heavily armored vehicles to save soldiers from roadside mines. Gates similarly pushes the bureaucracy to upgrade care for wounded soldiers and their families after years of neglect.

Yet he has nothing but praise for Bush, especially because he ordered a “surge” of 20,000 additional troops into Iraq in 2007 to avert a “potentially catastrophic” military defeat that his own administration had begot. Gates admired Bush as “a man of character, a man of convictions, and a man of action.”

Obama gets far harsher treatment. Gates is “blindsided” and “irked” when Obama announces plans to repeal the Don’t Ask, Don’t Tell law and let gays serve openly in the military, a process Gates supports but still tries to slow-roll. He is “disgusted” by the distrust that grows between senior military leaders and Obama, but he blames micromanaging by White House aides, not blunders by his own commanders. He questions whether Obama is fully committed to his strategy of sending more troops to Afghanistan, even though Obama rejected the advice of all his political advisers and sent 30,000 extra troops (and kept them there twice as long as Bush did in Iraq).

“Obama did the right things on national security, but everything came across as politically motivated,” he gripes.

Although Congress is an easy target these days, it must have been cathartic to write that “up close it is really ugly.” The House has “more than its fair share of crackpots” and “raving lunatics.” He derides “hypocritical and obtuse” members of the Senate and is furious at the “kangaroo-court environment” in hearings. He decries “rude, insulting, belittling, bullying, and all too often highly personal attacks” by lawmakers. Some, he adds, “suffered from some sort of mental duress that warranted confinement or at least treatment for anger management.”

Gates details his role at length in budget battles, personnel spats and inter-agency turf wars. Most of this is familiar ground. But it does yield some amusing details, as when Gates recounts his outrage at Obama closing an Oval Office meeting by warning “those of you writing your memoirs” that he had not made a decision. “I was offended by his suspicion that any of us would ever write about such sensitive matters,” Gates huffs … in his memoir.

Gates doesn’t share much about his management style, other than to say he played his cards close in meetings and policy debates so others wouldn’t know his views. That way, he didn’t have to commit, and could maintain leverage, until the end. He seeks to “turn down the temperature of the debate,” and in most cases he succeeds. But he seethes inside. He considers resigning or storming out of congressional hearings several times in furious protest. He never does.

The most moving passages appear when Gates describes his love for the troops and his attempts to hold his emotions in check as he signed deployment orders, visited hospitals and attended funerals. Each night, he hand-wrote condolence letters to the families of young men and women killed in the wars. And each night, he says, he wept. Toward the end of his tenure, he could barely speak to troops in the field without tearing up. “I realized I was beginning to regard protecting them – avoid their sacrifice – as my highest priority. And I knew that this loss of objectivity meant it was time to leave.”

Gates hints broadly that he is done with government. But who knows? He and Hillary Rodham Clinton, he notes, “agreed on almost every important issue.”

Bob Drogin, deputy Washington bureau chief for the Los Angeles Times, is the author of “Curveball: Spies, Lies and the Con Man Who Caused a War.”