“Duty: Memoirs of a Secretary At War” by Robert M. Gates; Alfred A. Knopf (640 pages, $35)

During 4½ years as secretary of Defense under presidents George W. Bush and 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 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. But it is compelling.

Gates is a foreign policy realist, so he views the world through relatively nonideological eyes. 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.

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.

Obama gets far harsher treatment. He 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.

“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.

Gates details his role at length in budget battles, personnel spats and interagency 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.

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.