The effect of Donald Trump’s sudden decision to pull all American special forces out of Syria could be electric.

An optimist — if any still exist when it comes to Syria — might tell you it removes another barrier to ending the country’s miserable, eight-year civil war.

But with so many competing interests, the likelier outcome is that the remaining factions will intensify a fight for the spoils in that wedge of Syrian territory in the northeast between Turkey and Iraq.

As is so often the case, the policy shift was announced by the US President came in a single, short tweet on Wednesday: “We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency.”

It is certainly true that Isis is no longer a force that holds substantive territory or operates institutions that loosely resembly government agencies — two things it did have earlier in this decade that allowed it to call itself a separate country.

The 2,000 US ground troops hitherto stationed in Syria were crucial in defeating the extremists: by aiding, arming and training the People’s Protection Units (YPG), a local Kurdish force, they helped drive them out of northern Syria’s principal towns and villages.

But to brand Isis “defeated” is a different matter altogether. The group still holds a slither of Syrian territory close to the Iraqi border and there is every reason to believe, says Bicom, the Middle East think tank, that it will emerge as an insurgent force in 2019.

By declaring “job done” and pulling American forces out, Mr Trump risks simply opening a new front for Turkey, which says the YPG is a terrorist organistion.

But what concern is all this for Israel, over 250 miles away from Syria’s northeastern territories? It boils down to Iran.

This week’s US withdrawal creates a vacuum. Turkey could fill it: President Erdoğan has spoken in recent weeks of using military force to disloge the Kurdish force and establish a “safe zone” similar to the one already created by the Turkish army in northwestern Syria. An awkward encounter between US and Turkish troops — NATO allies, remember — is now less likely.

But what if that northeast Syrian vacuum is instead filled by Iran?

Tehran has 3,000 personnel deployed in Syria and has established a local milita. Bicom reports Iranian companies have won civilian infrastructure contracts as Bashar Assad’s government rebuilds after eight years of civil war.

If an Iranian-backed Bashar Assad seeks to reassert control in the region, that would embed Iran’s grip on Syria — which is precisely what Israel wants to prevent.