Although the Iraq war is over, many perils remain that can destabilize the country: factional struggles for political power and economic resources, disputed internal boundaries in the north, separatist aspirations in the south, marginalization of minorities, and renewed sectarian and ethnic polarization. Add to this list menacing social movements such as the Sadrist Trend. These can exploit and exacerbate Iraq’s problems, challenging the government’s legitimacy and the country’s stability.

After the fall of Baghdad in 2003, Moktada al-Sadr emerged as the leader of the Jaish al-Mahdi Army (JAM), a small militia mobilized to resist the foreign occupation. As its reservoir of supporters rapidly increased, the JAM augmented its military wing with political, social and public-affairs offices and transformed into a potent movement, the Sadrist Trend. To understand the threat of the Sadrist Trend, it helps to look to Lebanon, where Hezbollah has grown from a ragtag militia into a large-scale political and social movement with effective military infrastructure and lethal terrorist capacity. It has destabilized neighboring countries, inflicted substantial losses on the region’s most powerful military during the 2006 Israel-Hezbollah war and caused the collapse of Lebanon’s unity government earlier this year.

Like Hezbollah, the Sadrist Trend is becoming a well-organized, entrenched part of the Iraqi polity. With the Iraqi government unable to provide adequate security and services, al-Sadr has stepped in with action and rhetoric that resonate with the Shia masses. He is following the same playbook that Hezbollah used to gain strength in Lebanon.

Yet Iraq may not go down the Lebanese path. Unlike Hezbollah, the Sadrist Trend lacks a reliable external patron in Iran. Rising tensions between Moktada al-Sadr and Tehran, together with escalating intra-Shia violence in Iraq, reveal that the group does not always share Iran’s agenda. This creates an opportunity for the United States to capitalize on the intra-Shia cleavages and counterbalance Iran’s destabilizing interference in Iraq.

The Rise of Moktada al-Sadr

Similar factors—centuries-long oppression of Shia, poor socioeconomic conditions and foreign occupation—gave rise to two Islamist movements: Hezbollah in Lebanon in 1982 and the Sadrist Trend in Iraq in 2003. Systematic persecution made the Lebanese and Iraqi Shia susceptible to appeals from radical groups and leaders who promised change and challenged the quiescent Shia politicians. All that was needed was a catalyst. For Lebanon, it was the civil war (1975–90), which led to the occupation by Israel in 1982. For Iraq, it was the U.S.-led invasion of the country in 2003, which created political and security vacuums resulting in ethnic and sectarian violence. These developments prompted the disenfranchised Shia communities in each country to mobilize to redress their long-standing grievances.

While political and socioeconomic conditions caused the rise of Hezbollah and the Sadrist Trend, the politico-religious ideology of resistance and discourse stressing change and justice enabled both groups to sustain support from their Shia base.

After his Jaish al-Mahdi Army was beaten by coalition forces in 2004 and again by the Iraqi Security Forces in 2008, al-Sadr changed tactics—turning from outright violence to political resistance. By stressing Iraq’s territorial integrity, decrying foreign occupation, and calling for social and economic reform, al-Sadr gained credibility among war-weary Iraqis. Like Hezbollah in Lebanon, the Sadrist Trend provides protection, arbitrates neighborhood disputes, restores electricity and water supplies, and offers other essential services to the Shia community. Al-Sadr has increased his appeal by backing up his rhetoric with social services that have produced tangible benefits in people’s lives.

The Sadrist Trend has evolved into an organized, mature force, while Moktada al-Sadr has gained enough political leverage to resolve the crisis that followed the 2010 parliamentary elections. Furthermore, al-Sadr has burnished his religious credibility: Though not yet a senior cleric (ayatollah) authorized to issue religious edicts (fatwa), he extends to his followers guidance on religious as well as political matters. Al-Sadr’s stature and pattern of leadership are reminscient of those of Hezbollah leader Hasan Nasrallah.

The Implications of the Sadrist Trend’s Power

The Sadrist Trend poses a number of risks stemming from its strong political and military infrastructure, its ability to control parts of Iraq’s territory and population, and its capacity to mobilize and deploy collective violence.

Its ability to “outgovern” incumbent leaders tarnishes the government’s legitimacy. Moreover, because the Sadrist Trend, like Hezbollah, has not only sought to delegitimize the government but also has held seats in parliament, it can play both sides. It can use its political power to destabilize the government of Prime Minister Nuri al-Maliki and promote anti-American policies while offering an outside alternative. Equally important, the existence of alternative power centers can impel incumbents to act in a dictatorial fashion. Given its history, it is plausible that Iraq will slip back into authoritarianism.

When militant groups struggle for the support of the same constituency, the result often is violence. One of Hezbollah’s largest military campaigns was directed against a rival Shia movement, Amal. In Iraq, Asa’ib Ahl al-Haq (AAH), which previously operated like a terrorist organization, is becoming a social movement as it seeks to increase its influence among Shias. It has opened religious schools to expand its pool of recruits and now asserts that AAH targets only “the occupation forces” and deems “the blood of Iraqi citizens and military personnel a red line.” This poses a challenge to the Sadrist Trend. In response, al-Sadr has denounced AAH members as “criminals and murderers,” and there have been frequent armed clashes between al-Sadr’s Promised Day Brigade and AAH.

The mounting tensions indicate that violent struggles for power and resources will dominate Iraq after the United States concludes its peacekeeping mission by January 1, 2012.

What Can the United States Do?

Historically, effective resistance movements have had the support of a regional state or a great power that provided funding, training, intelligence, arms and sanctuaries. Iranian and Syrian patronage has been critical for Hezbollah. In Iraq, Iran is supporting various Shia militias, including Asa’ib Ahl al-Haq, the Promised Day Brigade and Kata’ib Hezbollah.

But the growing tensions between the Sadrist Trend and Iran are evident in violent clashes between al-Sadr’s forces and other Iranian-backed Shia groups. Further, al-Sadr has publicly criticized Tehran, saying that “Iran is trying to expel its opponents from Iraq while it refuges the opponents of the Iraqi people.” He has denounced Iran’s support for his adversary, Ismail al-Lami (known as the Butcher of Baghdad), who committed atrocities against the Sunni Arabs when ethnic and sectarian violence engulfed Iraq in 2006.

The United States should exploit these cleavages to counter Iran’s influence in Iraqi society and weaken groups such as the Sadrist Trend and AHH. When the Sunni Arabs turned against al-Qaeda in Iraq in 2006–7, the United States took advantage of those rifts and succeeded in its counterinsurgency effort. Critically, effective strategy and adequate force levels and resources were key to America’s success in increasing Iraq’s stability.

Although the United States no longer can rely on its military, the U.S. diplomatic corps and intelligence and counterterrorism contingent should remain focused on Iraq and build on what the military has achieved. A clear demonstration of America’s commitment to the Iraqi people’s security and prosperity will be crucial.

Most Iraqis resent Iran’s interference, but without a viable alternative they might tolerate it. Similarly, in the absence of adequate security and essential services, Iraqis might strengthen their support for movements like the Sadrist Trend.

In 2004-5, coalition authorities dismissed Moktada al-Sadr and his movement as “nothing more than a nuisance.” That was an inaccurate assessment. The United States only has to look 450 miles to the west of Iraq to understand why it should not blunder by underestimating al-Sadr’s potential again.

Irena L. Sargsyan is a research analyst at the Saban Center for Middle East Policy at The Brookings Institution.