In what will likely mark the worst foreign-policy decision of his presidency to date, President Trump on Wednesday announced that he is ceding Syria to the jackals.

“We have defeated ISIS in Syria, my only reason for being there,” the president tweeted, confirming his decision to withdraw some 2,000 American troops from the war-ravaged country.

Those Yanks are currently based mostly in the eastern and northern parts of Syria, fighting ISIS remnants and training Kurdish and other ­allied fighters.

Mr. Trump’s closest Washington ­allies panned the decision almost immediately. “Withdrawal of this small American force in Syria would be a huge Obama-like mistake,” tweeted Senator Lindsey Graham.

Remember President Obama’s fateful 2013 decision to forgo enforcement of that infamous “red line” against use of chemical weapons by the Damascus regime? Just as then, leaving Syria now would mark a turning point in the war — for the worse — while delivering another blow to America’s prestige and global power.

Wednesday’s decision contradicts numerous previous administration statements.

“We’re not going to leave [Syria] as long as Iranian troops are outside Iranian borders, and that includes Iranian proxies and militias,” National ­Security Advisor John Bolton said in September.

Last week, the president’s special envoy to the anti-ISIS coalition, Brett McGurk, told State ­Department reporters that “it would be reckless if we were just to say, well, the physical caliphate is ­defeated, so we can just leave now. I think anyone who’s looked at a conflict like this would agree with that.”

Well, apparently not everyone.

So what made Trump decide, against all advice, to turn away from America’s Syrian commitments?

Earlier this week, Mr. Trump was on the phone with Turkey’s president, Recep Tayyip Erdogan, who recently threatened to invade northern Syria to fight Kurds controlling large swaths of the Turkish-Syrian border. That phone call may well have triggered Mr. Trump’s decision.

Turkey and America have a lot to discuss. Ankara, a partner in a project that created the world’s most formidable fighter jet, the F-35, made a deal with Russia to buy S-400 anti-aircraft missiles. America wants the Turks to instead buy American-made air-defense systems, and this week ­approved a $3.5 billion Patriot missile sale to Turkey.

Ankara, meanwhile, wants Washington to extradite Mr. Erdogan’s ­bogeyman, Fatullah Gulen, an Islamic preacher who resides in Pennsylvania. America has long resisted extradition, arguing that Turkey has no credible evidence for Mr. Gulen’s responsibility for a failed coup attempt against Mr. Erdogan in 2016. Yet on Tuesday, Trump’s press secretary, Sarah Sanders, said America will “take a look at” coughing up Mr. Gulen.

Mr. Trump’s decision to leave Syria, then, could be one component in a complex deal with Mr. Erdogan. Without American troops in the area, the Turks get a freer hand in dealing with the Kurds, while America gets — what exactly? It seems “The Art of the Deal” writer blundered in making major concessions to ­Ankara upfront without getting enough concrete commitments in return.

Meanwhile, the Kurdish fighters, who were America’s most reliable allies in chasing ISIS from Syrian territory, are left to the whims of Mr. Erdogan, who considers them anti-Turkish terrorists and who isn’t known for meting out mercy.

Israel’s prime minister, Benjamin Netanyahu, said Wednesday that Mr. Trump and Secretary of State Pompeo had informed him of the decision to leave Syria, making clear “they have other ways of expressing their influence in the area.” Mr. Netanyahu added that Jerusalem will study the withdrawal’s “timetable, how it will be implemented and — of course — its implications for us.”

Mostly, the implication is the removal of a major military bulwark against Iran and Hezbollah, which are trying to make permanent their presence in Syria, right on Israel’s border — a problem Mr. Bolton ­acknowledged in the recent past. Israel can’t be happy.

Earlier in the week, Mr. Netanyahu touted Israel’s successes against Iran in Syria, adding, “We’re striking Iran [militarily] in Syria and of course the United States struck them economically.”

He may have stumbled on a top Trump foreign-policy principle: America will fight in the economic arena, leaving physical combat to allies and partners.

When, though, we abdicate military ­responsibilities, as Mr. Trump has ­decided to do in Syria, the world’s most nefarious powers — in this case, Russia, Iran, the butcher Bashar al-Assad and Mr. Erdogan — take the spoils.

And yes, as numerous members of Mr. Trump’s administration have warned, ISIS could soon follow in one form or another, as well. This time, Trump won’t be able to blame his predecessor.

This column first appeared in the New York Post.