There is a significant change in U.S. strategy in Syria as the civil war there sputters to an end.  U.S. officials now say that the U.S. will keep some kind of military presence in the country until interested parties are able to broker a deal to end the conflict.  President Trump has said in the past that he wants to withdraw all U.S. military forces as soon as possible.

But aggressive Iranian moves to establish a permanent presence in Syria as a way to threaten Israel have forced a change in U.S. policy that is designed to check Iran's expanding influence in the region.

Washington Post:

"The president wants us in Syria until that and the other conditions are met," Jeffrey told reporters Thursday, saying the U.S. withdrawal was also linked to achieving a lasting defeat of Islamic State militants.

Jeffrey spoke days after national security adviser John Bolton announced that the United States would not withdraw "as long as Iranian troops are outside Iranian borders," for the first time tying the U.S. trajectory in Syria to challenging Iran. 

The new strategy raises the stakes for the Trump administration in Syria, where it must navigate an array of obstacles that also include Russian support for Syrian President Bashar al-Assad, which has reduced his incentive to make concessions required to end the fighting.

Iran is unlikely to easily relinquish its foothold on the Mediterranean after a decades-long investment it dramatically expanded after the war began in 2011.

Peace is not coming to Syria anytime soon, even if Syria-Iranian-Russian forces annihilate rebels in and around Idbib Province – their last stronghold.  The rebels have vowed never to give up, and terrorist militias are certainly not going to be party to any peace deal.

Therein lies the danger of this new policy – it's open-ended with nebulous goals.  That Iran wants to threaten Israel with an eye to some kind of future showdown with the Jewish state is well known to the Israeli government, and the Israelis can certainly take care of themselves.  But Iran setting up bases in Syria is only part of the Iranians' overall Middle East strategy.  From Syria, the Islamic nation threatens not only Israel, but Turkey, Yemen, Lebanon, and Iraq – all nominal U.S. allies.  Checking Iranian influence then becomes a necessity for the stability of the region.

The key are Iran's proxy forces:

The Trump administration has made countering Iran's powerful network of proxy forces, from Lebanon to Iraq to Yemen, a primary goal in the Middle East.  In Syria, Iran's Revolutionary Guard Corps is believed to command at least 10,000 fighters, including Shiite militiamen and government soldiers, forming the backbone of a force that has helped Assad claw back vast areas of the country from rebels. 

Faysal Itani, a Middle East scholar at the Atlantic Council think tank, said U.S. officials appear to have renewed hope that the long-stalled U.N. negotiating process can finally produce a settlement.  Or, he said, they may be preparing for a lengthy on-the-ground mission, given the remote likelihood of a deal anytime soon.

Those proxy forces are expensive to maintain.  It is thought that Iran gives Hezb'allah about $250 million a year.  Similar amounts are dispensed to the 100,000 Shia militiamen in Iraq and the Houthi rebels in Yemen.  They are also propping up the Assad regime in Syria. 

It is hoped that U.S. sanctions will economically strangle Iranian support for these proxy forces, making them less effective.  But this will take a while, given European reluctance to assist the U.S. in enforcing sanctions against Iran.

Iran presents a challenge unlike any other in the post World War II world.  Countering their drive for hegemony in the region will occupy U.S. planners for the next several decades.