The US assassination of Qassem Soleimani means that the United States and Iran are at war.

This is just a simple analytic truth, one obvious to experts. The killing of another country’s most important military official is tantamount to a formal declaration of hostilities. While the US and Iran had been exchanging blows indirectly and through proxies in Iraq and Yemen, the Trump administration has brought this long-running shadow conflict with Iran out into the open.

The question now is not whether the two countries are at war. It’s what kind of war they’re about to wage — and how many people are going to die as a result.

Neither the US nor Iran appears to want a full-scale conflict, meaning an extended US bombing campaign inside Iran’s borders or a ground invasion. Such a conflict would be devastating to both sides. However, when two enemies like these start openly shooting at each other, neither side wants to be seen as the one who blinks first. The result is a cycle of attacks and counterattacks, which has the potential to spiral outside of anyone’s control.

Iran’s supreme leader has already vowed some form of retaliation, and the many American personnel in the region are bracing for something bad. “We are sitting on the fence of either fleeing or hunkering down and riding out the wave of hell Iran will bring,” says a US official posted at the Baghdad embassy. “But we have scotch still.”

When the Iranian response comes — and it will come, though it may not be immediate — there will be intense pressure on the Trump administration to respond in kind. The scenarios experts are floating are dire, including both direct attacks on the US and strikes on its allies.

“Iran, for my money, will certainly try to assassinate a US-backed head of state or major figure,” Hussein Banai, an Iran expert at Indiana University, tells me. “If I’m the Gulf monarchies, I’m beefing up my security detail.”

While the desire on all sides is to avoid a full-scale war, it’s possible that responses of this caliber could pile on top of each other, goading each side into climbing up what military types call “the escalation ladder” — all the way to a full-scale war. A lot depends on what Iran does and how Trump personally reacts, given the functional power of the presidency to launch wars. These are variables we simply can’t weigh accurately now — and that should scare all of us.

“A lot of people are confident that this conflict won’t escalate out of control, while others seem certain that it will. Both groups are overconfident,” says Bear Braumoller, a political scientist at Ohio State University. “The truth is, escalation is unpredictable and can be very dramatic. We were confident that Hitler could be contained, right up to the fall of France.”

Why this situation is so damn scary

It’s worth noting that the killing of Soleimani did not come out of the blue.

In December, Iranian-backed Iraqi militias (functionally commanded by Soleimani) killed a US contractor at a base in northern Iraq. The US responded with an airstrike on the militias, in turn leading to militiamen storming the US embassy in Baghdad on New Year’s Eve. All of that came amid the backdrop of months and years of low-level hostilities, ranging from an Iranian attack on oil tankers in the Strait of Oman to US support for Saudi Arabia in its war with Iranian allies in Yemen to each party backing opposed sides in Syria (to deeply varying degrees).

The attack on Soleimani is different from all of these things. This is a direct action: the killing of one of the most important figures in Iran, in both military and political terms, rather than an attack on anyone’s proxies or allies. The open nature of the strike — with the US not only quickly claiming responsibility for the bombing, but also explicitly naming Soleimani as the target — represents a massive escalation above and beyond anything we’ve seen before.

“Soleimani had been in the crosshairs of US military several times but was spared. National security officials during the Obama era were always concerned such a move would lead to Iran activating terror cells around the world in retaliation,” the BBC’s Suzanne Kianpour reports. “I’m told Trump aware, still made [the] call.”

So why do it? Both the Pentagon’s official statement and Secretary of State Mike Pompeo labeled it an attempt to establish deterrence: to both disrupt Iran’s planning of future attacks on the US and to let them know that any such violence will be met in kind.

“General Soleimani was actively developing plans to attack American diplomats and service members in Iraq and throughout the region,” the Pentagon said. “This strike was aimed at deterring future Iranian attack plans.”

This theory has been labeled “deterrence by escalation” by some international relations scholars: the idea that inflicting greater pain on a military adversary will cause them to back down. The problem with this approach, though, is that an opponent who’s attacking you has already shown they’re willing to attack you. Iran’s December attacks on American positions in Iraq didn’t deter the US from attacking Iran; in fact, it prompted American retaliation. There’s every reason to assume that Iranian policymakers will react in the same way to this kind of massive escalation.

“Deterrence by escalation usually doesn’t work. It’s a classic form of underestimating the adversary,” writes Lindsay Cohn, a professor at the Naval War College. “Iran can’t win a conventional war with the US, but it can certainly impose costs.”

Given Iran’s key sources of military strength — its battle-tested ground troops and many proxy forces throughout the region — the possibility for what these costs look like are practically endless. Iran could kill American troops stationed in Iraq, launch attacks on global oil infrastructure in the vital Strait of Hormuz, use its many proxies in places like Lebanon and Syria to kill nearby Americans, launch terrorist attacks internationally, or kill leading American allies.

These attacks might not be limited to military targets. Iran could strike civilians in Europe or Americans in Latin America. Iran also has some limited capabilities to affect the US mainland; an Iranian cyberattack temporarily shut down Atlanta’s municipal government last year.

The US will not be able to predict or prevent escalation on all of these fronts. With an Iranian escalation all but guaranteed, the question is how severe it will be — and how the US will choose to respond in turn.

How the US-Iran tensions could spiral out of control

Everything — and I mean, everything — now hangs on the way the key decisionmakers in the two capitals handle this dynamic. The optimistic take is that the Iranian response will be limited, as the last thing it wants is an American invasion of Iran and regime change.

“Iran does not want war with the United States,” writes Georgetown Professor Matt Kroenig. “The Supreme Leader and his advisers are likely looking for a Goldilocks response, to strike back but not too hard.”

But the Iranians are cross-pressured here: They will want to show they’re uncowed by America’s attack. Hardliners in Iran’s government will want to strike back hard; there were mass demonstrations in Iran on Friday against the US attack. It’s possible that whatever response the Iranians feel is necessary to signal their own resolve might well be “too hard” for the Trump administration to tolerate, prompting yet another American response and keeping both sides climbing up the escalation ladder.

This is how you get a full-scale war that neither side wants. Each hopes to cow the other with violence, but in actuality keeps provoking the other until they’ve crossed a line that nobody is willing to back down from. Given the absolute horror that would be a US-Iran war, the decision to kill Soleimani was a gigantic risk.

I don’t want to be entirely alarmist. Historically, accidental wars are relatively rare. Leaders tend to find ways to pull back from the brink while saving face.

But neither Ayatollah Khamenei nor President Trump are particularly trustworthy decisionmakers, to put it mildly. It’s very hard to predict how they’ll handle this crisis, and things could easily get out of hand. The US-Iran war could well get much worse — and the mere possibility should scare all of us.

“I think this is a moment of reckoning for the Islamic Republic,” says Suzanne Maloney, an Iran expert at the Brookings Institution. “Anyone who tells you they know where it’s going is probably overconfident about their own powers of prediction.

Alex Ward and Dylan Scott contributed reporting to this piece.