A guiding line runs through the unpredictability and seeming chaos of President Trump’s foreign policy, summed by his favorite hashtag: #EndEndlessWars. That’s what pushed the president to announce, following a phone call with Turkey’s Islamist strongman Recep Tayyip Erdogan, the withdrawal from Northern Syria the few dozen American GIs who until now prevented Turkey’s invasion of its neighbor.

Mr. Trump appeared eager to fulfill a campaign promise to end wars. In that, as much as both would hate the comparison, he is no different from his predecessor, Barack Obama. Both have made their biggest foreign policy errors because they bowed to the yearning so widely-held in the wake of the Iraq war to take America out of the Mideast and its endless wars. Hence the hash tag.

Ending wars may be an admirable notion, espoused by beauty pageant contestants, Yoko Ono, and even Secretaries General of the United Nations. American presidents, though, should be more tethered to the real world: if pulling American GIs out equals no-war, how come we still have more American troops at Germany and Japan 75 years after World War II than we currently deploy in the Mideast and South Asia combined?

The fact is that, in the real world, aspiring to abruptly “end wars” often makes future wars more likely. That’s what we see at northern Syria. Our GIs, whose presence until last week prevented war, are now helplessly watching a bloodbath. Fox News’s intrepid Pentagon correspondent, Jeniffer Griffin, quotes a special forces soldier working with Syrian Kurds as “I am ashamed for the first time in my career.”

The history of America’s alliance with the YPG, the Kurdish fighters in northern Syria, is a bit more complex than portrayed by some otherwise excellent writers that now blame it all on Mr. Obama’s desire to appease Iran and to refrain from antagonizing its ally, President Bashar Assad. Mr. Obama initially tried to find “moderates” among Syria’s anti-Assad opposition groups.

Ruling out American boots on the ground, Mr. Obama hoped to turn these groups into ground proxies to fight ISIS jihadists. Yet in October 2015, the $500 million training program, run by the Pentagon and overseen by Mr. Obama’s point man in Syria and Iran, Brett McGurk, was suspended. That happened after Syrian trainees in Jordanian and Turkish camps ended up selling to jihadists arms they received from America.

It was only then that Mr. McGurk settled on the YPG, a Kurdish fighting force with ties to separatists in Turkey, the PKK. The YPG, along with Muslim and Christian Arabs and other Syrian minorities, proved to be an effective, reliable fighting force against ISIS. It quickly became a close American ally, proving to be the only capable ground force in the war, which Mr. Trump now says defeated ISIS “100 percent.”

The YPG and its Syrian allies suffered more than 10,000 casualties in that war. Now those Kurds, seen by Ankara as terrorists, bear the brunt of Turkey’s ire. Mr. Erdogan decided to launch his long planned Syria invasion this week, after Mr. Trump announced the withdrawal of American GIs from the region (most of the Yanks remain in northern Syria, but are now mere spectators as the bloodbath commences.)

The Turks bombard Kurdish-held positions in Syria with artillery and air power. Ankara-allied ground forces, mostly Arabs affiliated with militant Sunni groups, do most of the fighting on the ground. Mr. Erdogan defined his goal as creating an 18 mile “safety zone” inside Syria to prevent cross-border Kurdish attacks. The Kurds have vowed to defend the territory they consider theirs, much of it wrested away from ISIS.

With no American troops as buffer (and, as European diplomats tell me, Europe won’t even contemplate replacing them with its own soldiers), this is a recipe for renewed battle. Many of the players on the complex Syrian map were already appeased, as the Syrian war wound down. Now they are likely to find new cause for fighting. Hence, the desire to avoid war is ending up inviting it.

Then there’s the captured enemy. About 17,000 enemy fighters from ISIS and their families are held in facilities across northern Syria. A Kurd tells me that at night those YPG guards stay out of al-Hawl, the largest detention camp, fearing the Jihadists inside will capture and behead them. Yet, after ISIS lost all territory it controlled in Syria and Iraq, the victorious YPG managed to keep them incarcerated with nary an escape.

The facilities those Jihadists are held in are beyond the 18-mile zone Turkey plans to seize and hold. It has no plan to invade and hold the detention facilities. The YPG will abandon them, too. Mr. Trump said America is taking in two vicious ISIS prisoners, known as The Beatles, who have famously beheaded journalists. Europe may take in a minuscule number, but what about the rest?

Losing control over thousands of jihadi fighters would be bound to resurrect ISIS, or a similar group going by a different name. Mr. Obama has learned about the pitfall of “ending wars” the hard way. After announcing an election promise was fulfilled and pulling all troops from Iraq, he was forced to return there to fight a resurrected Jihadi force. Mr. Trump campaigned against Mr. Obama’s folly.

That is one of the reasons the reaction Trump’s latest decision is being met with such anger in the field and in the Congress. I means that Mr. Trump, or a successor if he fails to win the presidency next year, will learn the same lesson: “ending wars” artificially is the fastest and surest way to assure the next battle will be bloodier and more difficult to win.

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Mr. Avni, a contributing editor of the Sun, covers the United Nations.