Soleimani pictured in February 2016. Photo: STR/AFP via Getty Images

“The game has changed.”

Secretary of Defense Mark Esper on Thursday issued what, at the time, seemed to be a warning to Iranian-backed militias in Iraq. Within 12 hours, it turned into a prediction, with one crucial flaw — this is not a game. In less than a week, the standoff between the U.S. and Iran has zoomed from what seemed to be a somewhat calibrated exchange of rockets, cyberattacks, and rhetoric to the killing of a man reckoned to be Iran’s second-most-powerful military official, causing military and counterterrorism experts to worry about nasty scenarios from all-out regional war to terrorist retaliation against Americans abroad or at home.

Escalating tensions between the U.S. and Iran reached a flashpoint in recent days when Kataib Hezbollah, an Iranian-backed militia in Iraq and Syria, fired rockets at a base in Kirkuk Province, killing an American contractor and wounding several American servicemembers. The U.S. responded with air strikes that killed about two dozen militia members. Thousands of pro-militia protesters marched on the U.S. Embassy in Baghdad, trapping diplomats inside, vandalizing the compound, and burning the reception area before dispersing on Wednesday afternoon. President Trump said Iran would “be held fully responsible” for the attack on the embassy, and on Friday he followed through. A U.S. drone strike at Baghdad International Airport killed several officials from Iraqi militias backed by Tehran, along with a rather special Iranian visitor they were escorting: Qasem Soleimani.

As the head of the Islamic Revolutionary Guard Corps’s Quds Force, Soleimani was considered by many to be more powerful than the Iranian foreign minister. Your columnist cannot resist reminding you that in 2015, then-candidate Trump did not know who Soleimani was, had to be prompted in a friendly interview, and then confused the Quds Force (the IRGC’s secretive unit charged with clandestine operations) with U.S. allies the Kurds. At the time, Trump said the names “would all be changed” by the time he took office.

As the head of Iran’s elite paramilitary forces and the conduit to the allied militias Iran supports from Yemen and Iraq to Syria and Lebanon, Soleimani has the deaths of hundreds of Americans on his hands. Previous U.S. administrations had reportedly considered assassinating him and opted not to — because of the profound response such an action might have unleashed from Tehran and, perhaps, because he was believed to be a careful and strategic character whom Washington might revile but with whom it could also communicate.

Instead, here we are. On Friday morning, Iran’s supreme leader, Ayatollah Ali Khamenei, promised “a forceful revenge awaits the criminals who have his blood and the blood of the other martyrs last night on their hands.” While its unclear how exactly Tehran will retaliate, we can predict some of the broader consequences of this drastic escalation in U.S.-Iranian hostilities.

Utter political turmoil inside Iraq

Sunni Iraqis, and some of the young demonstrators who resent Iranian influence in their country, are rejoicing today. But their government depends on Iranian political and financial support even as it tries to balance the countervailing U.S. influence. If one or both sides try to force Iraq to choose, as they surely will, more violence and chaos — including more targeting of Americans — is the likely outcome. Commentators are predicting that Iraq may ask the U.S. to remove its forces, which would be a setback not just for the Trump administration’s effort to put military pressure on Iran, but also for its continuing effort to contain remnants of ISIS still active in the Iranian and Syrian borderlands.

More violence throughout the Middle East

Even before Khamenei promised “forceful revenge,” there was no doubt that Iran would retaliate for the killing of an official this senior, and it has plenty of options. Its proxies can target Americans and allied governments from Saudi Arabia to Israel. Hezbollah, in particular, is considered the world’s most effective non-state armed force; with Iran, it can surely go after Americans anywhere in the region. Before the sun was up Friday, the U.S. had issued a security alert instructing Americans to “depart Iraq immediately.” Meanwhile, Israel announced it was moving forces north to counter potential Iranian responses.

Potential terrorism beyond the Middle East

Over the decades, Hezbollah has pulled off some terrifying acts of violence far afield — perhaps most vividly, the 1994 bombing of a Jewish community center in Argentina that killed 85 people. It has also had some awkward failures, such as an alleged plot to blow up a Washington, D.C., restaurant frequented by the Saudi ambassador. How much should we worry about such attacks on U.S. soil? Far more than we did last week.

Political blowback at home and abroad

Soleimani’s killing is likely to have a significant impact on global politics. On Twitter, former Defense Department official Ilan Goldenberg suggested Iran may follow through on its threats to withdraw from the Nuclear Non-Proliferation Treaty and restart its bomb program in earnest.

Within the U.S., there were already questions on Thursday night about President Trump’s motivations for authorizing the strike, and how it might impact the upcoming election. For Americans of Middle Eastern origin, escalating tensions with Iran will likely increase fears of more restrictions on travel and immigration — along with more suspicion, humiliation, and prejudice at the border and in everyday life. There’s also the small matter of U.S. law and the Constitution. U.S. Senator Chris Murphy noted that Congress is required to approve assassinations of senior foreign officials — or any act of war that is not immediate self-defense.

The Pentagon said in a statement on Thursday night that Soleimani was “actively developing plans” to attack American diplomats and servicemembers throughout the Middle East, adding, “this strike was aimed at deterring future Iranian attack plans.” But many experts believe it will have the opposite effect. Ultimately, the night’s biggest casualty may be the hope that hostilities between the U.S. and Iran could remain small and manageable, without the kind of conflict and chaos that leads to major losses of life.

This post has been updated throughout.