Turkish forces are seen near Mount Barsaya, northeast of Afrin, Syria, on Tuesday.Khalil Ashawi / Reuters

What does it mean when NATO’s two biggest militaries find themselves on opposite sides of a conflict?

That’s what’s happening in Syria, where the United States and Turkey have for years differed over Washington’s backing of armed Kurdish groups Turkey sees as terrorists—and where that rift took on a violent new dimension last weekend. Following a U.S. announcement—later hastily walked back—that the United States was building a largely Kurdish security force on Turkey’s border, Turkey launched an offensive into Syria. By Wednesday, when Turkish President Recep Tayyip Erdogan declared the offensive would continue until the “terrorist” threat was eliminated, he said more than 200 Kurdish fighters had been killed, along with seven or eight Turkish troops.

The confrontation is an outgrowth of Syria’s chaotic battlefield, with its dizzying array of alliances that has at different times put rivals on the same side and friends on the opposite. Now that ISIS is all but vanquished, one organizing principle of the conflict has disappeared, re-exposing the divides combatants had de-emphasized to jointly fight the Islamic State. In this case, the key players are three countries—the United States, Turkey, and Russia—and one rebel militia, the Kurdish People’s Protection Units, known as the YPG.  The U.S. and Russia are geopolitical rivals, though the YPG has been allied with both. Turkey and Russia have come to blows over Syria in the past—notably in 2015 when Turkey shot down a Russian jet near its border with Syria—but Russia appears to have tacitly facilitated the Turkish incursion.  And the U.S. is allied with both Turkey and the YPG, who are now fighting each other.

So America is friends with Turkey’s enemy, but trying to remain Turkey’s friend, but Turkey is befriending a U.S. enemy. The Trump administration has been muted on the Afrin operation: It has praised the YPG’s role in defeating ISIS in Syria, but maintained that Turkey has legitimate security interests given its conflict with Kurdish separatists in its own country. The U.S. messaging has created yet more confusion. Secretary of State Rex Tillerson last week said the U.S. would maintain a long-term presence in Syria; the military said a border force including the YPG was part of that plan; and then Tillerson denied that any border-security force was planned at all.

James Jeffrey, a former U.S. ambassador to Turkey who is now at the Washington Institute, said Turkey generally supports America’s goal to stabilize Syria and prevent ISIS’s return, but it was infuriated not to be consulted about the border force. In the Turks’ eyes, the “apparent [U.S.] priority was to defend [the] Syrian border from… the Turks,” he wrote in an email. This even though the Afrin region saw repeated incursions into Turkey by Kurdish militants. Jeffrey continued: “Turks didn’t go into Afrin solely because of these ... developments, but their interest in sending a message to U.S.—‘we are serious’—probably was heightened by these … screw ups.”

Marc Pierini, a former EU ambassador to Turkey who is now a visiting scholar at Carnegie Europe, said in an email that Turkey also has domestic political reasons to launch the operation—Erdogan is facing re-election next year. “However,” he wrote, “the Afrin military operation is illustrative of diverging priorities between two NATO members, the U.S. and Turkey.”

That divergence could reach a dangerous point if Turkey expands its operations to other towns in Syria where U.S. special operators are working with Kurdish fighters—though that outcome is seen as unlikely. But the Atlantic alliance already had a Turkey problem. The country’s relations with Western European countries, many of which are NATO members, have been strained since a failed coup attempt against Erdogan in 2016; the Turkish president said he believed the West did not do enough to support him. In one notable episode last year, Erdogan likened the German leadership to Nazis for not letting him hold campaign events in the country, which has a large Turkish population. Russia, meanwhile, has been mounting its own effort to divide NATO from the outside, and is only too happy to differences in Syria or elsewhere.

The New York Times wrote: “In the judgment of Turkish, Greek and even American diplomats ... NATO’s southern flank is still as open as a wound a month after the Turkish invasion of Cyprus.” Ultimately, the alliance survived, though Cyprus remains divided more than four decades later. Greek-Turkish tensions still occasionally flare up, over issues like the fate of Turks who sought asylum in Greece after the coup attempt against Erdogan, and Turkish violations of Greek airspace.

As for America and Turkey, that relationship too has weathered repeated disputes. Turkey, for example, declined to allow U.S. troops passage through its territory during the 2004 invasion of Iraq—because of the overwhelming domestic opposition to the war. (It did allow supplies to pass.) In 2016, Turkey cut off power to  its air base at Incirlik—from which the U.S. was flying raids against ISIS—in the wake of the coup attempt, prompting concerns about the security of U.S. nuclear weapons there. Turkey’s role in Syria has not always furthered U.S. interests. As Steven A. Cook wrote last week in The Atlantic: “Over the course of the conflict in Syria, the Turkish government turned a blind eye to jihadists, enabled al-Qaeda affiliates, and was (at best) ambivalent about fighting the Islamic State.”

For its part, the United States has also seen strains with other alliance members members—including opposition to the Iraq War among members in the Bush years; revelations of U.S. spying on allies in the Obama years; and equivocation about the collective-defense commitment—which stipulates that an attack on one is an attack on all—in the Trump years.

In all of these cases, the alliance has held together. Pierini, the former EU ambassador, said the situation in Afrin does not in itself constitute a long-term threat to NATO, but if Turkey proceeds with operations east of the Euphrates River, where the U.S. has control, “this would heighten the risk of direct encounters with U.S. special forces present there.”

He added: “This would only benefit the Assad regime and Russia, and possibly [ISIS] remnants.” And it would put additional strain on the key commitment to treat an attack on one as an attack on all.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.