Jim Mattis is not an abstract kind of guy. He grew up rough and tumble — a trait we share — and he found order and meaning for his life in work, as did I. He did not resign over abstract policy differences with President Trump on alliance management and multilateralism. He resigned because the president endangered the lives of American troops for no good reason.

This much we know: Mattis never expected, intended or accepted that he would be driven to resign from the Trump administration. He told friends as much when they teased that he must keep a letter of resignation tucked inside his jacket pocket at all times. Unspoken but hanging in the air at such moments was the Mattisonian thought: The bastards will have to fire me.

He abased himself — not something he would have endured willingly — by staying on in a Cabinet of crooks, dolts and sycophants who form the biggest swamp in Washington in my 50-plus professional years here.

But Mattis maintained a sense of honor through it all — almost. Early on, while Vice President Pence and others fell over themselves to lavish praise on Trump in a command performance of an ersatz Cabinet meeting staged for TV cameras, Mattis just said it was an honor to serve the men and women of the nation’s armed services. I never caught him bending his own strict code of conduct toward the troops — until last month.

That was when he went along with Trump’s decision to send American troops to the Mexican border to counter an alleged invasion by a caravan of desperate would-be immigrants. I don’t need insider accounts to know that Mattis choked on using troops as part of Trump’s base political ploy. But at least the troops sent to Texas were not exposed to danger.

The same cannot be said for Trump’s irrational, boastful, abrupt public announcement of his decision to withdraw the 2,000 or so American troops now in Syria’s war zone. Informing the world, including the Islamic State and Syrian dictator Bashar al-Assad, of the shape of troop movements even before a plan for getting those troops out safely has been drawn up, was the last straw of treacherous action for Mattis. Telegraphing withdrawal while they are still in the field exposes troops to attack by the enemy, or from previously friendly forces now likely to feel betrayed, and would complicate hopes of reinforcement in extremis.

Mattis is fond of calling Washington a “strategy-free zone” no matter who is in the White House. (Mattis’s blunt arguments with Obama-era officials over Iraq got him fired as a theater commander, a move that probably boosted his chances of getting the top job at the Pentagon with Trump.) Now Trump seems determined to make Mattis a prophet by operating without any strategic underpinning for his actions.

The Syrian withdrawal could — with difficulty — be justified as part of an overall realignment of U.S. forces in the region, as could Trump’s desire to cut in half the 14,000-troop presence in Afghanistan. But Trump has made it clear that he is acting for purely political reasons having to do with campaign pledges he made, as has been the case with his threats to shut down the government over border wall funding.

Such moves will have offended the core of decency that is Mattis’s most dominant personal characteristic. He would have been upset by the implicit betrayal of the Kurds (an old American habit) in Syria and of our Western allies who rallied to the American war in Afghanistan. But Trump’s cavalier treatment of U.S. troops in harm’s way would have pushed Mattis to the breaking point.

I saw his sense of right and wrong demonstrated repeatedly when we overlapped for four years as visiting fellows in the ivory tower setting of the Hoover Institution at Stanford University until 2017, when Mattis came back to the corridors of power in Washington.

During that time, I saw him exhibit human compassion that would have been extraordinary for anyone, but particularly for a 43-year Marine veteran nicknamed “Mad Dog” (the moniker was no doubt another attraction for Trump), and whose orders to his troops poised to attack a city in Iraq were: Be Polite. Be Professional. Have a Plan to Kill Everyone You Meet.

When another Hoover fellow experienced a sharp decline in health that would lead to his death, Mattis was unfailingly present, cheerful and supportive to the family. Somehow, I didn’t imagine that compassionate quality was something President Trump would see as a plus. Trump has once again proved me sorrowfully right in spades. Consider Mattis’s resignation a cause for national mourning — and dread.

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