Hejin and the Deir ez-Zor region are predominantly Sunni Arab but the United States has long favored supporting Kurdish-led fighters from farther north over local opposition combatants.

Following President Donald Trump's announcement, the SDF was said to be talking to the pro-Iran Syrian government and considering the release of more than 1,000 IS fighters it is holding. The Kurdish-led forces have repeatedly been accused by other Syrian opposition groups of collaborating with the regime.

Much of IS-held Hejin east of the Euphrates River was retaken in December by Kurdish-led Syrian Democratic Forces (SDF), backed by the international coalition. Villages and areas near the town remain under IS control, however, according to locals contacted by Al-Monitor.

The surprise announcement Dec. 19 that the United States plans to pull its roughly 2,000 troops out of eastern Syria strongly suggests that the already heavy influence of Iran-backed armed groups near the border will rise.

QAIM, Iraq — As minor pockets of Islamic State (IS) territory in eastern Syria are treated as of little consequence following the entrance of US-backed fighters into the long-besieged Hejin region, Iraq’s western Anbar province is facing up to emboldened pro-Iran armed groups nearby.

Meanwhile, reports of cross-border drug smuggling in the desert south of Qaim, with proceeds presumably topping up the coffers of armed groups on both sides of the border, warrant attention.

The road from Qaim to Rutba in the western part of Iraq’s Sunni-majority Anbar province — across the border from Syria’s Deir ez-Zor region — is controlled by non-local Shiite Popular Mobilization Units (PMUs), reportedly with a heavy presence of Kata’ib Hezbollah, designated as a terrorist group by the United States but long active in the fight against IS in Iraq and on the side of the regime in Syria.

The group is known for its secrecy but is closely linked to Iran and has fighters on both sides of the border.

The desert town of Rutba — often called "the most isolated town in Iraq" — is allegedly at the center of cross-border drug smuggling, including a major trade in the illegal amphetamine Captagon, Al-Monitor was told by Anbar security officials this month.

Rutba lies on an international highway, around 110 kilometers (68 miles) east of the Jordanian border and 300 kilometers (186 miles) west of Anbar's capital, Ramadi. It is also about 110 kilometers north of the Saudi border.

The US decision to pull its troops out of Syria is likely to affect the small contingent at the al-Tanf base well to the west of Rutba in Deir ez-Zor, near the border crossing with Iraq.

It was in that area that 300,000 Captagon pills were seized in May by Maghawir al-Thawra, a Free Syrian Army group in Deir ez-Zor.

The commander of the opposition group, defected officer Mohanned al-Talaa, told Al-Monitor that an Iraqi and a Syrian smuggler had been caught with an estimated $1.4 million worth of illegal substances.

Reports in recent years have held that the pro-Iran Lebanese Shiite militia Hezbollah and its allies are behind a regional rise in the production and distribution of Captagon.

Proceeds from the lucrative trade can be used to buy weapons and pay fighters’ salaries.

Stories abound, meanwhile, that even high-ranking army officials, including the head of the Badiya and Jazira (western Anbar) area, Staff Maj. Gen. Qassem al-Mohammadi, have been refused access to the Qaim-Rutba road by the non-local Shiite PMUs in control of it.

Several security officials told or confirmed to Al-Monitor on condition of anonymity that general’s southbound vehicle was turned back at a checkpoint in the desert below Qaim in recent months in a show of power by the PMUs.

Though "security reasons" are often cited by default, the perception voiced by the officials Al-Monitor spoke with is that the military commander of western Anbar is significantly weaker than the non-local Shiite PMUs, including Kata’ib Hezbollah.

Two attacks in October on the Akkaz gas field south of Qaim shortly after their handover from the PMUs to the army led local security officials to claim to Al-Monitor that the non-local PMUs were actually behind the attacks.

But the western Anbar commander for the PMUs, Qassim Musleh, told Al-Monitor in an interview Oct. 11 that the fault lay with the army’s weakness.

Speaking about non-local Shiite PMUs linked to Iran in the area, Col. Moussa Hamad al-Karbouly, the commander of the local Sunni PMU Aaly al-Furat Brigade, told Al-Monitor in an interview at his Qaim headquarters, "This is bigger than the Iraqi government. It can’t control this."

The Aaly al-Furat Brigade works closely with the Iraqi army in maintaining security in parts of the city and flanks the military's operations in the desert area north of the Euphrates River. The river cuts through Qaim.

Karbouly posited that a return of IS is likely if the root causes of IS’s initial rise are not addressed.

In mid-December, there were more women and children shopping and playing in Qaim's streets than there had been in previous months. While the city seems safe, the persistent lack of electricity a year after it was retaken from IS and a dire lack of jobs may pose significant risks if not dealt with.

The unclear chain of command and proliferation of various security forces in the city also continue to create confusion, local residents told Al-Monitor.

Many say IS cells are likely simply "sleeping" for strategic reasons international coalition airstrikes continue on IS logistics bases in the desert but garner little media attention.

Though its ranks of foreign fighters have been decimated, territory lost and original support base eroded, IS retains the capacity to conduct a resurgence of some sort, experts say.

A Pentagon report from August estimated the number of IS fighters still active in Syria to be between 13,100 and 14,500.

The latest geopolitical shifts in eastern Syria mean that pro-Iran groups are likely to gain more control there. What effect this will have on the potential of IS or other terrorist and insurgent groups to regroup and regain local support is unclear.