Withdrawal would undermine Trump’s Iran policy.

Iran’s supreme leader could not dream of a U.S. action that would undermine the administration’s policy toward Iran more than a withdrawal from Syria. Trump appears poised to leave the Iran nuclear deal in May and has repeatedly indicated his desire to have a tough posture toward the Islamic Republic. Indeed, the president in October unveiled a U.S. strategy that would involve working with allies “to counter the [Iranian] regime’s destabilizing activity and support for terrorist proxies in the region.” It simply does not make sense to leave the nuclear deal, impose more sanctions, and target the regime’s development and proliferation of missiles only to cede Syria to Iran.

Why? Syria is the key to Iran’s destructive ambitions in the Middle East. The Islamic Republic seeks a land bridge from Tehran to Beirut to build a sphere of influence that reaches from its Afghan borders to the Mediterranean Sea—a continuous corridor of political and military control from which Iran can strengthen its regional influence and weaken that of the United States. Iran also seeks to build a long-term military presence in Syria, from which it can threaten U.S. allies in the region like Israel, and influence politics in Iraq and Lebanon.

To these ends, the Iranian regime has used Syria’s conflict to entrench itself militarily and politically, primarily through an extensive network of Shiite militias. If the U.S. leaves Syria, there will be little in the way of Tehran’s expansion.