The killing of Qassem Suleimani, the Iranian commander targeted by an American strike Thursday night, is the most consequential act taken against the regime in Tehran in thirty years—even if we don’t know what those consequences will be. One thing is clear: we’re entering a dangerous period, in which the conflict between the two countries could easily spin out of control.

Suleimani’s biography as a pivotal figure in Iran and the region is well known. Since the late nineteen-nineties, he was engaged in trying to remake the Middle East to Iran’s advantage, directing his proxies to kill or dispatch anyone who impeded his vision of an Iranian-dominated sphere of influence stretching from Tehran to the Mediterranean Sea. He was remarkably successful, legendary even—certainly the most influential operative in the region in modern times. He was involved in sponsoring terrorist attacks, propping up despots like Bashar al-Assad in Syria, helping to assassinate at least one foreign leader—the Prime Minister of Lebanon, Rafik Hariri—and killing hundreds of American soldiers along the way. In the latter years of the American war in Iraq, Suleimani’s militias deployed a particularly bloody weapon against U.S. soldiers—the “explosively formed penetrator,” or E.F.P.—which tore through the armor of U.S. military vehicles and wreaked havoc on soldiers and marines. It was no small irony that he died on the road to the Baghdad International Airport, where so many Americans soldiers and Iraqis died by ambush.

Suleimani’s death is a heavy blow to the Iranian regime. He was not just the central figure in the country’s foreign policy and military; he was also considered a pillar of the Revolution itself. Since 1979, Iran has regarded its defense against foreign enemies, particularly the United States, as central to its survival. Suleimani’s vision of the region was formed in the nineteen-eighties, during the Iran-Iraq War, which left more than a million people dead and for which the Iranians, not entirely without reason, blamed the U.S. and its allies. Suleimani, a veteran of that war, vowed that nothing like it would happen to Iran again, and he built the Quds Force—a wing of the Revolutionary Guard—into a small, mobile army capable of waging asymmetric warfare against the country’s enemies, including the United States. When I asked Ryan Crocker, a veteran American diplomat, what motivated Suleimani, he said that it was love of country—and also something more visceral: “Nationalism drives him, and the love of the fight.”

U.S. officials from previous Administrations have said that Suleimani did not live as a well-guarded recluse like Osama bin Laden, in Pakistan, and the military could have killed him; but the U.S. decided that it was not worth provoking a large-scale retaliation. “Suleimani was lucky,’’ Meir Dagan, the former head of Mossad, the Israeli intelligence agency, once told me. “It’s important to be lucky.” (Dagan died in 2016.)

Suleimani has been replaced by his longtime deputy, but it’s not clear that any commander in Iran is his equal in guile or status. He was as skilled in diplomacy as he was on the battlefield, and as comfortable with diplomats as he was with front-line soldiers, whom he adored. And he had an especially close relationship with Ayatollah Ali Khamenei, Iran’s Supreme Leader. “A guy like Suleimani—they don’t have any more like him,’’ John Maguire, a twenty-three-year veteran of the C.I.A. specializing in the Middle East, told me. Maguire is one of the few Americans ever to come to face to face with Suleimani. They met in Baghdad, in 2004, when Iraqi politicians were trying to broker a rapprochement between the U.S. and Iran. “He had a command presence,” Maguire said. “He walked into the room and you could feel him.”

In Iraq, Suleimani had four deputies, who helped oversee the Shiite militias who have, most recently, been leading demonstrations against the American Embassy. The leader of one of those militias, Abu Mahdi al-Muhandis, of Kata’ib Hezbollah, died in the same strike as Suleimani. Muhandis has a long record of attacking Americans, too, beginning with the bombing of the American and French Embassies in Kuwait, in 1983. Kata’ib Hezbollah—an organization backed, trained, armed, and directed by Suleimani—is responsible for the deaths of scores of American soldiers in Iraq. Ever since the Trump Administration walked away from the nuclear deal signed under the Obama Administration, the U.S. and Iran have engaged in a series of provocative acts. By killing Suleimani, the Trump Administration has risked a wider, more unpredictable conflict, which could flare in many places and in many ways. It’s hard to imagine that the Iranian regime won’t respond to the American strike—it will feel that it has to. But where, and how? Maguire told me that the Quds Force has long specialized in two tactics: hostage-taking and truck-bombing. But the Americans are so well fortified in Iraq (and across the Middle East), and the American military presence in Iraq is so robust, that it’s more possible that the Iranians, if they decide to retaliate, will do so elsewhere. “It’s a better bet that they will choose another place—somewhere where the Americans are not as well protected,’’ he said. Maguire told me that he is not convinced that the Iranians will respond right away, or even at all, because of the deep sense of shock of losing Suleimani. “It’s a body blow to the regime,’’ he said. The biggest danger, of course, is that the Iranians respond, and possibly miscalculate, and then the United States does the same. That’s how wars start.

President Trump said on Friday, as he has previously, that the U.S. is not seeking war or regime change in Iran. Yet, since taking office, the Trump Administration has made regime change its implicit policy. By pulling out of the Iranian nuclear accord and imposing crippling sanctions on the country, Trump’s advisers have wagered that they can bring the regime down. By killing Suleimani, the Administration has taken the fight directly to its leadership.