President Trump deserves immense credit for reassessing the evidence and retaining 400 U.S. troops in Syria. Russia, Iran, and Syrian dictator Bashar Assad, however, will react furiously to his decision.

Still, the first point to make here is that Trump's decision is absolutely the right one. Split between the garrison at At Tanf, close to the Jordanian-Iraqi border, and a garrison in Kurdish-controlled northern Syria, U.S. forces will sustain the momentum against the Islamic State while protecting America's Kurdish and Sunni tribal allies from Iranian, Russian, and Syrian aggression.

The ISIS concern is front and center here because absent U.S. forces, those tribes would have found themselves caught between ISIS and Iranian forces/militias. Offering the tribes protection against Iranian domination, ISIS would have found new sanctuary for revival and plotting against the West. Similarly, were the U.S. to have abandoned the Kurds, President Recep Tayyip Erdogan of Turkey would have brutally crushed them while largely ignoring ISIS. That would have led to a humanitarian disaster and, more importantly, a collapse of American credibility as an ally to be trusted through good times and bad.

But as I said, Russia, Iran, and Assad will be enraged at Trump's decision.

Russia's fury will take root in its lessened ability to act as the key power player between Syrian and Iranian interests on one hand and the interests of the Sunni Arab kingdoms and Israel on the other. Russian President Vladimir Putin sees this interlocutor role as the ingredient for new investments in Russia's economy and the undercutting of U.S. regional interests. Albeit, via supposedly deniable proxies, the Russians will almost certainly launch a near-term attack on U.S. residual forces in an effort to alter Trump's decision here. The U.S. can counter this threat by replicating the Mattis solution and offering to annihilate any threats to U.S. personnel.

Assad's fury is equally understandable. With U.S. forces remaining on his soil, the dictator knows that America will be able to influence the Syrian political process in a way that ensures opposition interests find a voice at the negotiating table. Assad had previously believed that America's troop withdrawal would give him restored dominion under the one-sided Astana political track. Trump's decision here will protect innocent civilian lives in places like Idlib and force Assad to negotiate in better faith. Though, it should be noted, Assad is unlikely to leave power anytime soon.

Then, there's Iran.

The Iranian hardliner fury here will fix on one key concern: the At Tanf garrison's disruption of the Revolutionary Guards' arms and logistics land bridge between Iran, through Iraq, and into southern Syria. This bridge is the instrumental ingredient of Ayatollah Ali Khamenei's effort to dominate Baghdad and Beirut, and to threaten missile attacks against Israeli population centers. To be clear, Trump's troop retention in Syria will restrain Iranian aggression and protect Israeli lives. Trump should ask Israel to end its cozy relationship with China in reciprocity for his decision here.

Ultimately, Trump has made the right call here. While 400 forces might not seem like a lot, they'll offer facilitation for other U.S. personnel in Syria and continued action against ISIS. Detering Russia, Iran, and Syria, these forces will also foster Syria's improved multisectarian stability. That stability is the best and only way to ensure ISIS 2.0 doesn't return.