The United States is on the brink of war with Iran. In fact, that war might have already begun. A U.S. drone strike early Friday local time in Iraq killed Maj. Gen. Qasem Soleimani — the architect of Iran’s grand strategy in the Middle East — and several of his Iraqi counterparts, including the prominent militia leader and paramilitary official, Jamal Jaafar Ibrahimi (also known as Abu Mahdi al-Muhandis).

Soleimani was the most influential Iranian official in the region, often representing his country in meetings with foreign officials, especially in Iraq, Syria and Russia. He was revered as a national hero by Iran’s regime, and often called a “living martyr” by Iran’s supreme leader.

Iraq’s pro-Iranian militias considered Soleimani their beloved patron, and their devout loyalty made him arguably the most powerful man in Iraq. Iran, and Iran’s proxies in Iraq, will almost certainly respond to his assassination with violence. War, in one form or another, will be the probable result.

So how did we get here?

Soleimani’s assassination followed a week of tit-for-tat attacks between Iraqi militants loyal to Iran and U.S. forces. Last Friday, 30 unguided rockets landed inside the K1 military base in Kirkuk, killing a U.S. civilian contractor and seriously wounding several U.S. and Iraqi troops. American officials blamed Kataib Hezbollah, the militia headed by Muhandis, which was also believed to have been behind a number of previous (but nonlethal) rocket attacks over the past few months. In response, the United States conducted airstrikes on five Kataib Hezbollah bases in Syria and Iraq, killing 25 and wounding dozens more.

Iraqi officials strongly condemned the U.S. action, which U.S. officials legitimized as a defensive effort necessitated by the Iraqi government’s inability to prevent the attacks and unwillingness to punish the culprits. Kataib Hezbollah members laid siege to the U.S. Embassy in Baghdad the next day, and promised to camp outside embassy grounds until U.S. forces left the country. The militants’ protest camp lasted less than 24 hours. They departed Wednesday after Kataib Hezbollah officials claimed to have made a deal with Iraq’s acting prime minister to work through parliamentary means to expel U.S. forces.

Escalation on both sides

The embassy incident is part of a larger pattern of escalatory behavior by Iran in response to the Trump administration’s maximum pressure campaign. The Trump administration withdrew from the multi-nation Joint Comprehensive Plan of Action (JCPOA) nuclear deal with Iran in 2018 and followed up with unilateral sanctions aimed at crippling Iran’s economy.

For sanctions to be removed, the Trump administration demanded Iran adhere to a list of demands (as described by Secretary of State Mike Pompeo in May 2018), which would require Iran to completely abandon its most important strategic programs, from ending support to regional proxies to curbing its ballistic missile program.

President Trump suggested in numerous tweets and statements that he might be satisfied with less by emphasizing his desire to enter into talks with Iran and strike a better deal. Iran has refused to compromise.

Instead, Iran opted to push back through hard power. Over the past several months, Iran and its proxies engaged in a number of escalating attacks aimed at compelling a shift in U.S. policy. In the spring, Iran attacked tankers of U.S. Arab allies in the Persian Gulf, and in September, suspected Iranian rockets and drones struck oil facilities in Saudi Arabia.

Iran’s leaders were thus at a crossroads: either seek compromise with Washington through talks or keep escalating. Iran decided on the latter, which led to this week’s escalatory violence in Iraq.

So what will Iran do now?

The assassination of Soleimani will provoke Iran to act. Iran’s supreme leader, Ayatollah Ali Khamenei, has vowed “harsh revenge.” But its options are limited.

Iraq is probably where any Iranian response will begin. One option will be to push for the expulsion of U.S. forces, which Iraqi politicians may welcome after the killing of Muhandis and other militia officials.

Such a move would benefit Iran strategically in three ways: It would reduce U.S. influence in Iraq, lessen the U.S. regional footprint, decrease the U.S. ability to strike Iranian and allied targets, and at least temporarily make the United States the focal point of Iraqi politics again. The latter two reasons might be enough for Iran to pursue this goal.

However, expelling U.S. forces would also be risky for Iran and its allies. The Iraqi government and Iranian-backed militias both heavily rely on U.S. air power in combating the Islamic State. Although the Islamic State’s ability to control territory in Iraq has almost entirely collapsed, it remains a potent force and has maintained a steady stream of attacks. The absence of U.S. air power will inevitably allow the Islamic State a greater degree of space to operate.

With Iraqi forces already stretched thin, the absence of U.S. military support would sharply increase the likelihood of the Islamic State remobilizing in the country. Neither Iran nor the militias it supports have shown any ability to effectively combat the Islamic State on their own.

A renewed Islamic State threat would present yet another challenge to the Iraqi state, which is already plagued by endemic unrest, Kurdish separatism, a simmering anti-government Shiite protest movement, and now a renewed proxy war between the United States and Iran.

Further, U.S. forces have been a key point of leverage for Iran. From 2004 to 2011, Iran used its ability to kill U.S. soldiers through its proxies such as Kataib Hezbollah and Asaib Ahl al-Haq as a way to constrain U.S. action against Iran. During that period, all U.S. policy toward Iran was constrained by Iran’s ability to kill Americans in Iraq. With the return of U.S. forces to Iraq in 2015, Iran regained that leverage.

Iran’s proxies have been vowing to strike U.S. forces ever since, using such threats as a constant reminder that any action against Iran could lead to retaliation against U.S. troops in Iraq. Last week, Kataib Hezbollah made good on those threats.

The strike on Soleimani was described by Secretary of Defense Mark T. Esper as a deterrent attack, meant to discourage future Iranian attacks on U.S. forces. If attacks subside, then the assassination will have proved effective, and Iranian leverage over U.S. forces on the ground will have been lost. This makes a passive response unlikely.

Iran and its allies are more likely to find ways and opportunities to kill Americans in the region. That doesn’t mean that they will pursue full-scale warfare, but they will look for revenge to recapture leverage and scuttle U.S. efforts to deter.

Iraq’s months-long protest movement, which has shown no sign of abating despite the killings of hundreds of protesters by pro-Iranian militias and security forces, has been starkly critical of Iran’s influence in the country. By shifting attention back to the United States, Iran might seek to overwhelm anti-Iranian sentiment by stoking renewed anger at the United States.

Without a U.S. footprint in Iraq, Iran will be the sole remaining outside power in the country. This will make it even more difficult for Iran to skirt blame for its contribution to Iraq’s present miseries. Should Iran succeed in driving out the Americans, it might come to regret it.

Iran has no easy options. We are thus entering a dangerous new phase in the U.S.-Iran conflict.

Afshon Ostovar is an assistant professor of national security affairs at the Naval Postgraduate School and a Fox Fellow at the Foreign Policy Research Institute.