James Mattis exits, along with the guardrails for Donald Trump's foreign policy

Global conflict was so common and catastrophic early last century that world wars were numbered. But in more than 70 years there has been no World War III, in large part because of what Secretary of Defense James Mattis described in his resignation letter this week as a  "unique and comprehensive system of alliances and partnerships" led by the United States.

Such coalitions triggered the collapse of the Soviet Union in 1990-91, drove the Taliban from power in Afghanistan after 9/11, and all but defeated the Islamic State in Iraq and Syria this year. America's post-WWII foreign policy expanded trade, opened new markets, and lifted hundreds of millions of people out of poverty and into freedom. 

But as the much-admired Pentagon chief makes clear in his letter, President Donald Trump sees the world very differently, and the two men are irreconcilably at odds. Mattis' view involves respect for allies and standing up to malign actors. Under Trump's "America First" view, allies are freeloaders, foreign aid dollars are better spent rebuilding at home and authoritarian leaders are to be admired, if not solicited.

That policy was on full display this week with Trump's Twitter announcement to pull U.S. troops out of Syria. Never mind that the relatively small footprint of 2,000 largely special operations soldiers has been vital to the still-unfinished business of vanquishing Islamic State remnants and providing a stable environment for the rebuilding necessary to prevent their resurgence.

The U.S. presence is also a bulwark against a troika of strongmen: Turkish President Recep Tayyip Erdogan, threatening violence against Syrian Kurds who fought alongside U.S. troops; Syrian President Bashar Assad, eager to expand his murderous hold on power; and Russian President Vladimir Putin, who supports Assad and cheered Trump's decision to withdraw. In addition, U.S. troops are a check on Iran's regional threat and the risks posed to American ally Israel.

Trump rejected all of it, apparently without consulting some key national security aides and ignoring the advice of others. White House spokeswoman Sarah Huckabee Sanders' promise in April that strategic decisions in Syria would be based on commanders and troops "on the ground" are now empty words.

Nor does Trump's campaign promise never to telegraph moves to an enemy hold any meaning after news this week that he's moving — in the midst of peace negotiations with the Taliban, no less — to reduce by half the 14,000 U.S. troops serving in Afghanistan.

That troubling war has lasted 17 years. But in September, Trump seemed to embrace a Mattis strategy to support America's allies in Kabul and send Taliban insurgents the clear message that the only future was negotiating an end to the violence. With Trump's abrupt decision to draw down troops, why should the Taliban agree to anything?

More than a year ago, Sen. Bob Corker, the Tennessee Republican who chairs the Senate Foreign Relations Committee, said the only people separating "our country from chaos" — by serving as guardrails for an unpredictable president — were Secretary of State Rex Tillerson, White House Chief of Staff John Kelly and Mattis.

Tillerson is gone. Kelly and Mattis are on their way out. The consequences for U.S. foreign policy going forward are chilling to contemplate, and Mattis is signaling that things could get worse before they get better. One by one, Trump is wearing down or driving away the few good men and women standing between him and his worst impulses.

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