Read: What Iran is really up to in Syria

Indeed, one of the biggest challenges stemming from Iran’s involvement in Syria is that it has deployed thousands of Shia militants throughout the country. These forces are composed of foreign nationals trained and equipped by Tehran to maximize the regime’s battlefield effectiveness while minimizing costs for Iranian troops. Many of these militias are now beginning to leave the Syrian theater. Iran reportedly promised many benefits to these men upon their return from the conflict. But with its own economic woes and growing discontent within its populace, the Iranian regime isn’t in a position to support thousands of these fighters, many of whom come from Afghanistan, along with their families.

Instead, the mullahs could be looking to redirect these well-trained, battle-hardened forces to other conflicts where it’s currently involved, like the wars in Afghanistan and Yemen. Reportedly, they have already begun deploying some of them to these other theaters. As a result, these troops aren’t just a threat to Syria, but to other combustible places in the region. The United States will likely need to determine how to discourage Iran from redirecting them. Yet with the U.S. withdrawal from the Iran nuclear deal, diplomatic relations have reached a nadir.

In crafting its containment strategy, one key factor for the United States is the role of Russia. The administration is rightly looking to leverage President Trump’s rapport with Russian President Vladimir Putin to influence Iran’s moves in Syria. But Russia’s ability to shape Tehran’s behavior is limited. The mullahs have refused to defer to Russia, their partner in the conflict, even when it has offered strategic guidance. From their perspective, they’ve earned their position there, and the right to operate with autonomy. The United States could, however, help enlist Russia to de-escalate tensions between key players in Syria, while taking care not to further empower Moscow in the process.

Perhaps the most worrisome source of tension is the brewing conflict between Iran and Israel. In recent months, the two enemies have been facing off in Syria, with Jerusalem striking Iranian positions in theater and Tehran and its proxies firing rockets and flying drones close to Israel’s border, perhaps to provoke an overreaction. Preventing a broader conflagration between them in Syria will be an essential part of the new Iran containment strategy. As such, it may be the only way to force Iran to commit to downsizing its military footprint in Syria in any meaningful way, making diplomacy a critical tool in the pursuit of this new policy. Keeping U.S. troops in Syria may be necessary to securing leverage in any future political settlement.

Read: The U.S. will spend billions in Syria—just not on rebuilding it.

The outcome of this policy pivot will have major implications for the region. If containment fails, Iran will have a clear path to extending its influence throughout the Middle East, while the United States could become further stuck in the morass of Syrian civil war with few tangible gains to show for its revamped approach.

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Colin P. Clarke is an adjunct senior political scientist at the RAND Corporation and a senior research fellow at the Soufan Center. Connect Twitter