The geopolitical consequences of the above can hardly be overstated. If Iran consolidates its control over Iraq – the country with the second largest population in the Persian Gulf and rich oil and gas resources—Tehran will be in a strong position to dominate the entire region.

Iran will want to eliminate the presence of external regional powers. As a rising hegemon, Tehran would see the presence of U.S. and other Western forces in the region as an obstacle to its goal of being the dominant regional power. If the Shiite minority Houthis consolidate their takeover of Yemen, Iran’s ability to threaten Saudi Arabia and the traffic in the Red Sea, an important strategic waterway for commerce, will also increase.

Iran is already in position to disrupt another key chokepoint—the Strait of Hormuz. With domination of Iraq, Iran will be in a strong position to intimidate the GCC states, and continue to easily supply the Assad regime in Syria and Hizballah in Lebanon. In Iraq itself, the Iran-backed militias are unlikely to disband even if IS is defeated. Instead, they will continue to maximize power and possibly take over the state. Iran will likely use them to keep unfriendly Shiites, Kurds and Sunnis in check. Iran might also use them to destabilize the Gulf states.

Tempting though it might be for some to believe that the U.S. can wash its hands of all the headaches in that neighborhood by coming to an arrangement with Iran and letting it take over the job of policing and stabilizing that troublesome region, this is not an option. The U.S. cannot bandwagon with Iran and accept its hegemony over Iraq and its primacy in a new regional equilibrium in the Gulf. Accommodating Iranian hegemony would mean U.S. abandonment of the Gulf. This would give Iran control over the vast resources of the region, which it will use to pose a still bigger threat to the rest of the region and beyond. The Gulf states, especially Saudi Arabia, are likely to resist accommodating Iran and if they lose confidence in the U.S., they might look to Russia and China to balance Iran. The Saudis, who invested considerable financial resources in the Pakistani nuclear program in the 1980’s and 90’s, may push Pakistan to transfer nuclear weapons to them so they can deter Iran.

The U.S. faces many constraints. It cannot deploy large U.S. ground forces to Iraq to lead the battle against IS directly and reduce the role of the militias by coercing them to disarm or become part of provincial guard forces. Given the extent of Iranian influence in Baghdad, the Iraqi government will not ask for a large-scale U.S. ground presence in Iraq. In any case, such a deployment would be risky for the United States. It would be expensive and endanger many lives all while lacking the needed support politically at home. 

Perhaps, the best option is to contain Iran. This means preventing further Iranian expansion, increasing the difficulties and costs to Iran of what it is doing in Iraq, Syria, Lebanon and Yemen.

Increased costs over time could lead to increased opposition inside Iran and force the leadership to reconsider its expansionist policies – and may even present opportunities for change in Iran. Iran is particularly vulnerable in Syria and Yemen—given that the forces Iran supports in both countries are minorities. In addition, the U.S. should reestablish the linkage between the lifting of sanctions and Iran’s behavior in the region.

In Iraq, a containment strategy would require elevating opposition to Iranian domination to the same level as the fights against IS and its affiliates and Iran’s nuclear program. This would require support for Shiite, Kurdish and Sunni forces hostile to Iranian domination and friendly to the United States and the West. Given Iran’s domination of Baghdad, the United States would need to reconsider its current policy of providing security assistance through the central government.

The United States should find ways to empower Iraqi Prime Minister Abadi by inviting him to Washington, press him to limit the scope of where and how the militias operate, and help him develop national institutions—particularly the Ministry of Defense—to balance and eventually outlaw the militias. Washington needs to also work with Abadi to implement the agreed upon decentralization and establishment of provincial guard forces, which some of the militias can be integrated into.

The United States should also strengthen ties with regional friends, many of whom publicly and privately express uncertainty about U.S. goals and strategies in the region. A credible balance of power must be established to deter and if necessary defeat further Iranian expansion.  

The recent changes in the Middle East, especially Iraq, necessitates a reexamination and adjustment of U.S. policies toward Baghdad, Tehran, and the wider region. Staying on the current course means accommodating Iranian domination of Iraq, and over time of the Gulf.

 Zalmay Khalilzad is a Counselor at the Center for Strategic and International Studies (CSIS). He was the US Ambassador to Afghanistan, Iraq and the UN.