President Trump was instantly tweeting tough about Bashar al-Assad’s latest atrocity, the gassing of civilians in rebel-held Eastern Ghouta. But America’s response this time should be more than Trump’s answer to Assad’s use of chemical weapons a year ago.

It’s time for Washington to declare a no-fly zone in Syria east of the Euphrates, the region Trump freed from ISIS occupation. Assad and his Russian and Iranian allies don’t deserve to win the fruits of America’s war there, nor should Turkey be allowed to take its insane vendetta against the Kurds into an area the Trump administration liberated.

If America pulls out, as Trump suggested earlier in the month, the forces who helped defeat ISIS and the civilians they now protect will be at the mercy of thugs just as brutal.

As Bloomberg’s Eli Lake notes, US declaration of a no-fly zone — and at least implicit guarantees of other action if needed — won’t be nation-building, nor risk getting bogged down in a quagmire. It’s simply a repeat of the first President George Bush’s action after the first Gulf War, when he declared similar protection for Iraqi Kurdistan.

That move let the Iraqi Kurds build a decent (if imperfect) free society outside Saddam Hussein’s grasp, which left them positioned to help the United States in the Second Gulf War and ever since. They remain a key ally in checking Iran’s influence in the region — while mainly taking care of themselves, at no risk to US troops.

A year ago Saturday, Trump launched 59 Tomahawk missiles at the airfield that hosted an earlier Assad chem attack. Plainly, the lesson wasn’t learned, or perhaps the dictator figured the president’s recent remarks about leaving Syria once ISIS is toast were a license to slaughter freely again.

Either way, Assad and everyone else in the region needs to know that Trump is no patsy, that America will stand by its friends and against barbarism. Another round of Tomahawks might prevent more chem attacks. But Assad — with help from Russia, Iran and the terrorists of Hezbollah — will commit plenty of atrocities with conventional weapons, as they have throughout this war.

And Iran will gain control of the entire northern Middle East, from Lebanon through Syria and Iraq — freeing it to push harder in Yemen and Bahrain and threaten a conflagration that will reach Saudi Arabia and the rest of the Gulf, as well as Israel.

At a minimum, as Lake warns, by completely pulling out, Trump “will be leaving the people who were liberated from ISIS to be slaughtered and displaced. He will be helping Iran complete its land bridge to the Mediterranean Sea. He will have committed American blood and treasure to advance the strategic aims of America’s enemies.”

Drawing a line at the Euphrates, by contrast, will create a refuge for those saved from ISIS and Assad & Co. — and a chance for those US-allied fighters to build a free society that checks Iran’s ambitions.

It may not be the post-ISIS endgame Trump hoped for, but it’s the only way not to hand the fruits of this US victory to America’s enemies — and the enemies of civilization.