President Trump has made a fateful and terrible decision in announcing that the United States will abandon the Kurds in northeastern Syria to the mercies of Turkey's autocratic leader, Recep Tayyip Erdoğan.

Trump's betrayal leaves our Kurdish allies open to the possibility, nay likelihood, of mass killings. It also sets up conditions for a wider geopolitical disaster. The winners are Turkey, Russia, Syria, Iran, and the Islamic State. The biggest loser, after the Kurds, is the U.S.

Trump is right that our foreign policy should focus on American security and that we should not be a global policeman. But with great power and influence, if a nation wishes to keep it, comes great and widespread responsibility. And the president is wrong, in any case, to suppose that abandoning the Kurds won't undermine our security.

At issue is Rojava, the Kurdish stronghold in Syria. Half a decade ago, this was ground zero of ISIS's caliphate. Over the past three years, however, a U.S.-armed coalition, mostly of Kurds, has expertly evicted those murderous fanatics from all major territorial bases and crushed most of its dwindling number of jihadis. For most of this operation, one of the most effective in U.S. military history, no more than 2,000 Americans have served in the theater, with fewer than 1,000 there now. Since Trump took office, eight Americans have died in Syria; two of those were not in combat and one was from Turkish shelling.

The U.S. has provided support in the way of technology, arms, logistics, and tactical leadership. Almost all of the actual fighting has been done by the Kurds, a culturally unified and impressive warrior people without a state of their own.

The Kurds also were brave allies against Saddam Hussein, and to our shame we abandoned them then, too, allowing the Iraqi dictator to massacre them and triggering a massive exodus of refugees into neighboring countries.

Turkey regards the Kurds as mortal enemies and insurgents, and Ankara has designs on Rojava. This is not entirely unjustified; Kurdish separatists, not formally related to those fighting in Rojava, have conducted terrorist attacks within Turkey at various points in the last several decades.

Under Erdoğan, Turkey has been more an adversary of the U.S. than an ally, strategically aligning with both Russia and, in effect if not in intention, with Iran. The Kurds in Rojava are thus a bulwark not just against ISIS but against perfidious action by all three of those regimes unfriendly to America.

The Kurds, meanwhile, have been an invaluable ally. They have held prisoner some 10,000 ISIS fighters. It is far from certain that the Turkish government, with its own Islamist proclivities, will care as much about keeping them imprisoned and away from renewed malignancy in the region.

Trump is sending the message that Washington is an unreliable ally, willing to cut and run not just from losing allies such as the South Vietnamese in the 1970s but from winning ones, too. When vital American interests are involved, it will now be far more difficult for other nations to take the risk of siding with the U.S. This is lamentable.

And it is not ameliorated by Trump's tweets Monday in response to the firestorm of criticism he received from all sides. He tweeted: "As I have stated strongly before, and just to reiterate, if Turkey does anything that I, in my great and unmatched wisdom, consider off limits, I will totally destroy and obliterate the Economy of Turkey (I've done before!)." Against considerable competition, this tweet might be the weirdest and least reassuring that our chief executive has ever written.

As a practical matter, much more American money will be squandered and many more Americans may die if we are forced to operate globally without allies, and if, as seems likely, we have to return to Iraq to sort out the deadly chaos that will ensue once we are gone.

It is no wonder that even Trump’s strongest political allies are denouncing his decision vociferously. Former U.N. Ambassador Nikki Haley tweeted, “We must always have the backs of our allies, if we expect them to have our back … Leaving [the Kurds] to die is a big mistake.”

Some of Trump's critics today are relentless hawks or dedicated humanitarian interventionists. We don't share those viewpoints. We agree with Trump's "America First" foreign policy. And it's on that score that we oppose this rash retreat.

America needs allies who can rely on us. We need the Kurds as allies in this dangerous region. Trump's action threatens to undermine the U.S. on both fronts and leave us worse off.