President Trump will soon have made good on his vow to crush ISIS in Syria and Iraq. But that success is going to force him to decide what comes next — whether to risk wider war by protecting America’s on-the-ground allies in Syria, or to leave them to the untender mercies of the Assad-Putin-Iran coalition.

Russia is already threatening to shoot down US planes over Syria, after an American craft took out a Syrian-regime plane that was assaulting units of the Syrian Democratic Forces — the US-armed ground troops who’ve been taking the fight to ISIS.

Iran is flexing its muscle, too, with missile strikes into Syria, even as Putin’s navy also fires inland.

Recall that Assad and Russia, until very recently, had never faced ISIS head-on. Indeed, the Syrian despot de facto allied with the terror group in several battles against Syrian rebel forces. Only now, with ISIS on the ropes, are they moving in — and only so that Assad can take the territory, as he aims to restore his rule over all of Syria.

Mainly, Russia (like Iran) has merely helped Assad slaughter the rebels. Putin means to keep his naval bases on the Syrian coast; Iran is intent on controlling (via allies and proxies) a vast crescent of territory from Lebanon through Syria and Iraq.

The US-allied forces now stand as the main barrier to those ambitions. Will we keep protecting them, once ISIS is driven from its last strongholds?

A point-blank, stare-him-in-the-eyes warning to Putin (along with credible threats of consequences) might get the Russians to back off: They don’t need Assad to regain all his power. But the Iranians and Assad won’t stop until they’ve tested US resolve — repeatedly.

Most US allies in the Mideast would be happy to see America protect the SDF as a check on Iran’s power. But not, sigh, Turkey — because the group is mostly Kurds whom Ankara fears will support the warring of Turkish Kurd rebels.

Indeed, the Turks say Washington promised to take back the arms we gave the SDF once ISIS is crushed. (How the heck Team Trump could enforce that is anyone’s guess.)

We understand Trump’s advisers are divided on what to do, since no choice is good here. The president is going to have to make this call — and stick with it.