One of President Trump’s final foreign policy decisions of 2018 was also among his most controversial: the withdrawal of the remaining 2,000 U.S. troops from Syria. The order was an astonishing reversal of U.S. policy, and it raised concerns among Washington national security professionals that the Kurds—who have served as U.S. allies in the fight against the Islamic State, or ISIS—will suffer losses while the Assad regime, Russia, and Turkey gain. This weekend, the president’s national security advisor, John Bolton, seemingly reversed course again, announcing that U.S. forces would remain in Syria until ISIS was defeated and the Turks provided guarantees that they wouldn’t strike the Kurds.

The actor who perhaps benefits above all others from the administration’s back and forth on Syria is Iran. An American withdrawal would provide the Iranians with the operational space to expand their growing network of Shiite foreign fighters, who can be mobilized and moved throughout the Middle East. The recent announcements send Tehran the message that Washington will no longer be an obstacle in the way of these designs. Indeed, according to Bolton, the administration’s preconditions for withdrawal have to do with the Kurds and ISIS: the national security advisor made no mention of the presence or expansion of Shiite militias trained and equipped by Iran.

IRAN IN THE GRAY ZONE

In the early days of the Islamic Republic, which marks its 40th anniversary in February, Iran’s clerical leaders often spoke about exporting their revolution beyond national borders. Although spreading revolutionary ideals is no longer among Iran’s core objectives, the Islamic Republic has nonetheless spent decades cultivating ties with groups—particularly Shiites—in countries it considers important to its security. These connections link Tehran to constituencies in Afghanistan, Iraq, Lebanon, Pakistan, Syria, and Yemen, among other places.

Starting with its cultivation of Lebanese Hezbollah, Iran has reaped immense benefits from waging proxy warfare and leveraging third-party combatants, including terrorists and militias. Working with non-state clients allows Tehran to deter its enemies, grow its strategic depth, and compensate for its conventional military inferiority by projecting power beyond its means. Importantly, these groups help extend Iran’s reach into so-called gray zones between peace and conflict, in countries such as Iraq and Syria, while also affording it a modicum of plausible deniability and minimizing the costs of intervention.

Iran has reaped immense benefits from waging proxy warfare and leveraging third-party combatants.

When the Syrian civil war started in 2011, Iran was determined to prop up its ally. But divisions in Damascus and Tehran militated against an overt and large Iranian presence in Syria. Many Iranians took issue with helping sustain a dictator whose mass atrocities and use of chemical weapons raised ethical questions and damaged their country’s reputation. The Assad regime, for its part, sought to give the country’s defense a homegrown veneer and to avoid the appearance of a weak central authority being rescued by foreigners. Instead of intervening directly, Tehran organized non-Iranian Shiite forces to fight on behalf of the Assad regime. To do this, Iran recruited heavily from Shiite populations in Lebanon, Iraq, Afghanistan, and Pakistan. While Iran has long cultivated forces from Lebanon and Iraq, the introduction of Shiites from South Asia was new. These foreign fighter subgroups are embedded firmly within a broader network, but they have specific designations: the Fatemiyoun division consists of Afghans, and the Zeinabiyoun brigade of Pakistanis. To date, the Iranian regime has trained, equipped, and deployed several thousand such forces to Syria, going as far as recruiting child soldiers.

While the existence of Iran’s Shiite foreign-fighter network is hardly newsworthy in itself, the U.S. withdrawal adds a new twist. Without a U.S. commitment to counter Iran in Syria, the network will prosper and grow, taking advantage of the power vacuum to extend Tehran’s influence throughout the region, long an Iranian objective that has been kept in check by the deployment of U.S. troops to Iraq from 2003 until 2011. A significantly reduced American military footprint—Washington is expected to keep several hundred special operations forces in the country—will likely prompt further Israeli action in Syria, since Jerusalem can no longer depend on American military might to deter Iranian expansion. In turn, this could tempt Iran to use the fighters it has been cultivating as an external strike force.

THE NETWORK BEYOND SYRIA

To recruit fighters to Syria, Iran doesn’t rely solely on religion. It has reportedly promised to provide returning fighters and their families with Iranian residency, pay, health care, and education. Now, with the war in Syria winding down, Tehran will look to replicate its success there in other theaters. It can minimize the cost by using forces it has already recruited, trained, and equipped, and whose effectiveness has been enhanced by battlefield exposure in Syria.

There is a dearth of precise and accurate data on Tehran’s spending to support militias. But at a time when the country has to tighten its belt to overcome economic sanctions, Iran’s cultivation of Shiite foreign fighters may be the most cost-effective way—even using the highest cost estimates—to pursue its regional agenda. As Iran scholar Afshon Ostovar put it, “Iran spends less—and perhaps far less—on defense than its nearest peer rivals in the region, even including its support for its clients in Syria, Iraq, Lebanon, and Yemen.”

Iran is willing to act as a stabilizing force even as the United States considers cutting its troop numbers in half.

With the American withdrawal from Syria, and in the absence of any coherent U.S. policy to respond to actions in the region that fall short of conflict, Iran is likely to continue cultivating Shiite militias and redirecting them across the region. Some of the Fatemiyoun are already being redirected to Afghanistan, where, alongside Russia, Iran is supporting the Taliban to stymie the Islamic State’s offshoot in the country. This commitment shows Afghans that Iran is willing to act as a stabilizing force even as the United States considers cutting its troop numbers in half. And Afghanistan is just one of the four theaters in which Iran is involved: Iraq, Syria, and Yemen are the others.

President Trump’s decision to withdraw American troops from Syria was undoubtedly intended to extricate the United States from the morass of the Middle East and afford Washington the flexibility to focus its resources on great-power conflict—particularly with China and Russia, but perhaps also with Iran. However, by outsourcing the counter-ISIS mission to Turkey and essentially ceding Syria to the Assad regime and its powerful backers in Moscow and Tehran, the administration may be unintentionally signaling that it is unwilling to compete in critical geopolitical hotspots.

Such a message could embolden powerful states—including Iran—to expand their presence. Tehran is likely to step into the breach by continuing to build its network of Shiite foreign fighters and moving them around the region. If Iran’s leaders see this as a worthwhile strategy whose benefits outweigh its costs, the network could grow from a regional threat to a global one, ultimately creating more problems in the long term for the United States and its allies.