Phew! "Turkey sends tanks into Syria ...," CNN headlined on Thursday. "The goal is to crush ISIS." It's about time Turkey joined the war against Islamist terror. Some had suspected Turkish president Recep Tayyip Erdogan of having a soft spot for ISIS, even of letting his country be used as a supply base. So what explains his newfound zeal? Is he chastened by the failed July 15 coup? Is it that, since the battles around Kobani almost two years ago, ISIS has suffered a string of reversals?

The answer is more troubling. As the German weekly Der Spiegel put it: "Isis is the pretext. The Kurds are the target." Turkey's strategic objective is not to "crush" ISIS. It is to crush the most effective part of the anti-ISIS coalition: the Syrian-Kurdish Democratic Union Party (PYD) and People's Protection Units (YPG). Kurdish nationalism has itself led frequently to terrorism over the years and is an abiding national-security obsession for the Turks. The Istanbul daily Sabah ran a headline Thursday that said nothing about ISIS. It read: "Turkish forces shell YPG positions in northern Manbij."

Turkey cleared out the ISIS-held town of Jarabulus, on its border. Had it not done so, the Kurds would have. Militias were moving north from the city of Manbij, which the Kurds seized earlier this month. The anti-ISIS part of the Turkish operation was over before it started. Le Monde interviewed a Jarabulus resident named Mohammed, who said that ISIS, by arrangement, had begun to withdraw two weeks ago. The group took hostages with them. The Kurdish news agency ANF called it a " Handover from ISIS to ISIS through the Turkish Army."

Kurdish Manbij, not Arab Jarabulus, is what worries the Turks, as is evident if you consult the map at the Ukrainian military-strategy website UA. Kurds control the eastern two thirds of Syria's border with Turkey, along with a western "canton" around the city of Afrin (the yellow patches on the map). And the Kurds are on the verge of ousting ISIS from its last (gray-shaded) redoubt on the Turkish border. This would unite the two zones into a contiguous Rojava (as they call Syrian Kurdistan) that runs from the oil-rich lands around Mosul almost to the Mediterranean.

It is to protect and broaden the ISIS-built buffer between the two Kurdish zones—not to respond to anything ISIS is doing there—that the Turks have invaded Syria. During a visit from Vice President Joe Biden that coincided with the invasion, the prime minister Binali Yildirim claimed U.S. authorization for doing so: "Our arrangement with the United States is that the Kurds must withdraw from Manbij and its region to the east bank of the Euphrates," he said. "That is the commitment, the guarantee, that the United States has given us."

Speaking in Stockholm and Riga this week, Biden proclaimed that the Turks have finally gotten serious about the threat of ISIS. "I think the Turks are prepared to stay in an effort to take out ISIL as long as it takes." But in Istanbul, he seemed to forget ISIS even existed: "No corridor, period. No separate entity on the Turkish border. A united Syria." As for Manbij: "We have made it absolutely clear to the elements that were part of the Syrian Democratic Forces, the YPG that participated, that they must move back across the river. They cannot, will not, and under no circumstances get American support if they do not keep that commitment, period."

The groups to whom he's reading the riot act have been the only consistently effective anti-ISIS force in the Middle East. Supporting Turkey's desire for their neutralization is a quo for which one can only assume America is somehow getting a quid. Perhaps it involves the European refugee crisis, which Turkey is helping stem, but at an ever-mounting price. Apparently that price is nobody's business but the Turks'.