A deadly opening attack. Nearly untraceable, ruthless proxies spreading chaos on multiple continents. Costly miscalculations. And thousands — perhaps hundreds of thousands — killed in a conflict that would dwarf the war in Iraq.

Welcome to the US-Iran war, which has the potential to be one of the worst conflicts in history.

The Thursday night killing of Maj. Gen. Qassem Soleimani, who led Iranian covert operations and intelligence and was one of the country’s most senior leaders, brought Washington and Tehran closer to fighting that war. Iran has every incentive to retaliate, experts says, using its proxies to target US commercial interests in the Middle East, American allies, or even American troops and diplomats hunkered down in regional bases and embassies.

It’s partly why the Eurasia Group, a prominent international consulting firm, now puts the chance of “a limited or major military confrontation” at 40 percent.

But the seeds of conflict weren’t planted with Thursday’s airstrikes alone. Washington and Tehran have remained locked in a months-long standoff that only continues to escalate. The US imposed crushing sanctions on Iran’s economy over its support for terrorism and its growing missile program, among other things, after withdrawing from the 2015 nuclear deal last year; Iran has fought back by violating parts of the nuclear agreement, bombing oil tankers, and downing an American military drone.

The crisis has become more acute over the past week. An Iranian-backed militia killed an American contractor while wounding others in rocket attacks, leading the Trump administration to order retaliatory strikes on five targets in Iraq and Syria that killed 25 of the militia’s fighters. In protest, the militia — Ketaib Hezbollah — organized a rally outside the US embassy in Baghdad where some got inside the compound and set parts of it ablaze.

That led Secretary of Defense Mark Esper to tell reporters on Thursday that “if we get word of attacks, we will take pre-emptive action as well to protect American forces, protect American lives,” adding “the game has changed.” The US killed Soleimani hours after that statement, underscoring that change.

Importantly, experts note that neither country wants a full-blown conflict, with President Donald Trump saying he prefers “peace” when it comes to Iran. But the possibility of war breaking out anyway shouldn’t be discounted, especially now that Iran’s leadership has sworn to avenge Soleimani. “The great nation of Iran will take revenge for this heinous crime,” Iranian President Hassan Rouhani tweeted Friday morning.

The flag of General Soleimani in defense of the country's territorial integrity and the fight against terrorism and extremism in the region will be raised, and the path of resistance to US excesses will continue. The great nation of Iran will take revenge for this heinous crime. — Hassan Rouhani (@HassanRouhani) January 3, 2020

Which means US-Iran relations teeter on a knife edge, and it won’t take much more to knock them off. So to understand just how bad the situation could get, I asked eight current and former White House, Pentagon, and intelligence officials, as well as Middle East experts, last July about how a war between the US and Iran might play out.

The bottom line: It would be hell on earth.

“This would be a violent convulsion similar to chaos of the Arab Spring inflicted on the region for years,” said Ilan Goldenberg, the Defense Department’s Iran team chief from 2009 to 2012, with the potential for it to get “so much worse than Iraq.”

How the US-Iran war starts

US-imposed sanctions have tanked Iran’s economy, and Tehran desperately wants them lifted. But with few options to compel the Trump administration to change course, Iranian leaders may choose a more violent tactic to make their point, especially after Soleimani’s death.

Iranian forces could bomb an American oil tanker traveling through the Strait of Hormuz, a vital waterway for the global energy trade aggressively patrolled by Tehran’s forces, causing loss of life or a catastrophic oil spill. The country’s skillful hackers could launch a major cyberattack on regional allies like Saudi Arabia or the United Arab Emirates.

Israel could kill an Iranian nuclear scientist, leading Iran to strike back and drawing the US into the spat, especially if Tehran responds forcefully. Or Iranian-linked proxies could target and murder American troops and diplomats in Iraq.

That last option is particularly likely, experts say. After all, Iran bombed US Marine barracks in Lebanon in 1983 and, according to the Pentagon, Iranian-backed fighters killed more than 600 US troops during the Iraq War. Taking this step may seem extreme, but “Iran could convince itself that it could do this,” Goldenberg, now at the Center for a New American Security think tank in Washington, told me.

At that point, it’d be nearly impossible for the Trump administration not to respond in kind. The recommendations given to the president would correspond to whatever action Iran took.

If Tehran destroyed an oil tanker, killing people and causing an oil spill, the US might destroy some of Iran’s ships. If Iran took out another US military drone, the US might take out some of Iran’s air defenses. And if Iranian-backed militants killed Americans in Iraq, then US troops stationed there could retaliate, killing militia fighters and targeting their bases of operation in return. The US could even bomb certain training grounds inside Iran or kill high-level officials.

It’s at this point that both sides would need to communicate their red lines to each other and how not to cross them. The problem is there are no direct channels between the two countries and they don’t particularly trust each other. So the situation could easily spiral out of control.

Messaging “is often more important than physical action,” Jasmine El-Gamal, formerly a Middle East adviser at the Pentagon, told me. “Action without corresponding messaging, public or private, could most certainly lead to escalation because the other side is free to interpret the action as they wish.”

Which means the initial tit-for-tat would serve as the precursor to much more bloodshed.

“What are we going to be wrong about?”

You may have heard the phrase “the fog of war.” It refers to how hard it is for opposing sides to know what’s going on in the heat of battle. It’s particularly difficult when they don’t talk to one another, as is the case with the US and Iran.

Which means that the way the US and Iran interpret each other’s next moves would mainly come down to guesswork.

Eric Brewer, who spent years in the intelligence community before joining Trump’s National Security Council to work on Iran, told me that’s when the Pentagon and other parts of the government rely heavily on their best-laid plans.

The problem, he noted, is that wars rarely play out as even the smartest officials think they will. A guiding question for him, then, is “what are we going to be wrong about?”

Here’s one scenario in which the US might get something wrong — and open up the door to chaos: After America launches its first set of retaliatory strikes, Iran decides to scatter its missiles to different parts of the country.

Now the Trump administration has to figure out why Iran did that. Some people in the administration might think it’s because Tehran plans to attack US embassies, troops, or allies in the region and is moving its missiles into position to do so. Others might believe that it was merely for defensive reasons, with Iran essentially trying to protect its missile arsenal from being taken out by future US strikes.

Messaging “is often more important than physical action” —Jasmine El-Gamal, former Middle East adviser at the Pentagon

Without a clear answer, which interpretation wins out comes down to which camp in the Trump administration is the most persuasive. And if the camp that believes Iran is about to launch missile strikes wins, they could convince the president to take preemptive action against Iran.

That could be a good thing if they were right; after all, they’d have made sure Iran couldn’t carry out those planned attacks. But what if they were wrong? What if the other camp guessed correctly that Iran was merely moving its missiles around because it was scared the US would strike once more? In that case, the US would have bombed Iran again, this time for essentially no reason — thus looking like the aggressor.

That could cause Iran to retaliate with a bigger attack, setting off a spiral that could end in full-scale war.

Iran could make a grave error too. Imagine Trump sends thousands of troops, say 25,000, along with advanced warplanes to the Middle East in the hope that they’ll deter Iran from escalating the conflict any further.

Tehran could just as easily read that buildup as preparation for a US invasion. If that’s the case, Iranian forces could choose to strike first in an effort to complicate the perceived incursion.

Of course, cooler heads could prevail in those moments. But experts say the political pressures on both Washington and Tehran not to be attacked first — and not to be embarrassed or look weak — might be too strong for the countries’ leaders to ignore.

“Unintended civilian casualties or other collateral damage is always possible, and it is not clear that this administration — or any administration — understands what Iran’s own red lines are,” El-Gamal, now at the Atlantic Council think tank in Washington, told me. “As such, the greatest risk of a full-blown war comes from one side miscalculating the other’s tolerance” for conflict.

If that proves true, and the US and Iran officially escalate their fighting to more than a few one-off attacks, it’s war.

What the US-Iran war might look like

At this point, it’s hard to be very precise about a hypothetical full-blown conflict. We know it would feature a series of moves and countermoves, we know it’d be very messy and confusing, and we know it’d be extremely deadly.

But unlike with the path to war, it’s less useful to offer a play-by-play of what could happen. So with that in mind, it’s better to look at what the US and Iranian war plans would likely be — to better understand the devastation each could exact.

How the US might try to win the war

The US strategy would almost certainly involve using overwhelming air and naval power to beat Iran into submission early on. “You don’t poke the beehive, you take the whole thing down,” Goldenberg said.

The US military would bomb Iranian ships, parked warplanes, missile sites, nuclear facilities, and training grounds, as well as launch cyberattacks on much of the country’s military infrastructure. The goal would be to degrade Iran’s conventional forces within the first few days and weeks, making it even harder for Tehran to resist American strength.

That plan definitely makes sense as an opening salvo, experts say, but it will come nowhere close to winning the war.

“It’s very unlikely that the Iranians would capitulate,” Michael Hanna, a Middle East expert at the Century Foundation in New York, told me. “It’s almost impossible to imagine that a massive air campaign will produce the desired result. It’s only going to produce escalation, not surrender.”

It won’t help that a sustained barrage of airstrikes will likely lead to thousands of Iranians dead, among them innocent civilians. That, among other things, could galvanize Iranian society against the US and put it firmly behind the regime, even though it has in many ways treated the population horribly over decades in power.

There’s another risk: A 2002 war game showed that Iran could sink an American ship and kill US sailors, even though the US Navy is far more powerful. If the Islamic Republic’s forces succeeded in doing that, it could provide a searing image that could serve as a propaganda coup for the Iranians. Washington won’t garner the same amount of enthusiasm for destroying Iranian warships — that’s what’s supposed to happen.

Trump has already signaled he doesn’t want to send ground troops into Iran or even spend a long time fighting the country. That tracks with his own inclinations to keep the US out of foreign wars, particularly in the Middle East. But with hawkish aides at his side, like Secretary of State Mike Pompeo, there’s a chance they could convince him not to look weak and to go all-in and grasp victory.

But the options facing the president at that point will be extremely problematic, experts say.

The riskiest one — by far — would be to invade Iran. The logistics alone boggle the mind, and any attempt to try it would be seen from miles away. “There’s no surprise invasion of Iran,” Brewer, who is now at the Center for Strategic and International Studies think tank in Washington, told me.

Iran has nearly three times the amount of people Iraq did in 2003, when the war began, and is about three and a half times as big. In fact, it’s the world’s 17th-largest country, with territory greater than France, Germany, the Netherlands, Belgium, Spain, and Portugal combined.

The geography is also treacherous. It has small mountain ranges along some of its borders. Entering from the Afghanistan side in the east would mean traversing two deserts. Trying to get in from the west could also prove difficult even with Turkey — a NATO ally — as a bordering nation. After all, Ankara wouldn’t let the US use Turkey to invade Iraq, and its relations with Washington have only soured since.

“It’s almost impossible to imagine that a massive air campaign will produce the desired result. It’s only going to produce escalation, not surrender.” —Michael Hanna, a Middle East expert at the Century Foundation

The US could try to enter Iran the way Saddam Hussein did during the Iran-Iraq war, near a water pass bordering Iran’s southwest. But it’s swampy — the Tigris and Euphrates rivers meet there — and relatively easy to protect. Plus, an invading force would run up against the Zagros Mountains after passing through, just like Saddam’s forces did.

It’s for these reasons that the private intelligence firm Stratfor called Iran a “fortress” back in 2011. If Trump chose to launch an incursion, he’d likely need around 1.6 million troops to take control of the capital and country, a force so big it would overwhelm America’s ability to host them in regional bases. By contrast, America never had more than 180,000 service members in Iraq.

And there’s the human cost. A US-Iran war would likely lead to thousands or hundreds of thousands of dead. Trying to forcibly remove the country’s leadership, experts say, might drive that total into the millions.

That helps explain why nations in the region hope they won’t see a fight. Goldenberg, who traveled recently to meet with officials in the Gulf, said that none of them wanted a US-Iran war. European nations will also worry greatly about millions of refugees streaming into the continent, which would put immense pressure on governments already dealing with the fallout of the Syrian refugee crisis. Israel also would worry about Iranian proxies targeting it (more on that below).

Meanwhile, countries like Russia and China — both friendly to Iran — would try to curtail the fighting and exploit it at the same time, the Century Foundation’s Hanna told me. China depends heavily on its goods traveling through the Strait of Hormuz, so it would probably call for calm and for Tehran not to close down the waterway. Russia would likely demand restraint as well, but use the opportunity to solidify its ties with the Islamic Republic.

And since both countries have veto power on the UN Security Council, they could ruin any political legitimacy for the war that the US may aim to gain through that body.

The hope for the Trump administration would therefore be that the conflict ends soon after the opening salvos begin. If it doesn’t, and Iran resists, all that’d really be left are a slew of bad options to make a horrid situation much, much worse.

How Iran might try to win the war

Retired Marine Lt. Gen. Vincent Stewart left his post as the No. 2 at US Cyber Command in 2019, ending a decorated four-decade career. Toward the end of it, he spent his time at the forefront of the military intelligence and cybersecurity communities.

If anyone has the most up-to-date information on how Iran may fight the US, then, it’s Stewart.

“The Iranian strategy would be to avoid, where possible, direct conventional force-on-force operations,” he wrote for the Cipher Brief on July 2, 2019. “They would attempt to impose cost on a global scale, striking at US interests through cyber operations and targeted terrorism with the intent of expanding the conflict, while encouraging the international community to restrain America’s actions.”

In other words, Tehran can’t match Washington’s firepower. But it can spread chaos in the Middle East and around the world, hoping that a war-weary US public, an intervention-skeptical president, and an angered international community cause America to stand down.

That may seem like a huge task — and it is — but experts believe the Islamic Republic has the capability, knowhow, and will to pull off such an ambitious campaign. “The Iranians can escalate the situation in a lot of different ways and in a lot of different places,” Hanna told me. “They have the capacity to do a lot of damage.”

Take what it could do in the Middle East. Iran’s vast network of proxies and elite units — like Soleimani’s Islamic Revolutionary Guard Corps — could be activated to kill American troops, diplomats, and citizens throughout the region. US troops in Syria are poorly defended and have little support, making them easy targets, experts say. America also has thousands of civilians, troops, and contractors in Iraq, many of whom work in areas near where Iranian militias operate within the country.

US allies would also be prime targets. Hezbollah, an Iran-backed terrorist group in Lebanon, might attack Israel with rockets and start its own brutal fight. We’ve heard this story before: In 2006, they battled in a month-long war where the militant group fired more than 4,000 rockets into Israel, and Israeli forces fired around 7,000 bombs and missiles into Lebanon.

About 160 Israelis troops and civilians died, according to the Israel Ministry of Foreign Affairs, and about 1,100 Lebanese — most of them civilians — perished, per Human Rights Watch, a US-headquartered advocacy organization. It also reports about 4,400 Lebanese were injured, and around 1 million people were displaced.

But that’s not all. Iran could encourage terrorist organizations or other proxies to strike inside Saudi Arabia, the United Arab Emirates, and other Gulf nations. Last year, it planned and executed drone strikes on two major Saudi oil facilities deep inside the kingdom, convulsing world markets. Its support for Houthis rebels in Yemen would mostly certainly increase, offering them more weapons and funds to attack Saudi Arabia’s airports, military bases, and energy plants.

Experts note that the Islamic Republic likely has sleeper cells in Europe and Latin America, and they could resurface in dramatic and violent ways. In 1994, for example, Iranian-linked terrorists bombed the hub of the Jewish community in Argentina’s capital, Buenos Aires, killing 85 people and injuring roughly 300 more.

That remains the largest terrorist attack in Latin America’s history, and the possibility for an even bigger one exists. In 2018, Argentina arrested two men suspected of having ties with Hezbollah.

But Chris Musselman, formerly the National Security Council’s counterterrorism director under Trump, told me the US and its allies may have the most trouble containing the proxy swarm in Western Africa.

“We could see a conflict that spread quickly to places the US may not be able to protect people, and it’s a fight that we are grossly unprepared for,” he said, adding that there’s a strong Hezbollah presence in the region and American embassy security there isn’t great. Making matters worse, he continued, the US isn’t particularly good at collecting intelligence there, meaning some militants could operate relatively under the radar.

“This isn’t really a law enforcement function that US can take on a global scale,” he said. It would require that countries unwittingly hosting proxies to lead on defeating the Iranian-linked fighters, with US support when needed.

The chaos would also extend into the cyber realm. Iran is a major threat to the US in cyberspace. Starting in 2011, Iran attacked more than 40 American banks, including JPMorgan Chase and Bank of America. The attack made it so the banks had trouble serving its customers and customers had trouble using the bank’s services.

In 2012, Iran released malware into the networks of Saudi Aramco, a major oil company, which erased documents, emails, and other files on around 75 percent of the company’s computers — replacing them with an image of a burning American flag.

In the middle of a war, one could imagine Tehran’s hackers wreaking even more havoc.

“We could see a conflict that spread quickly to places the US may not be able to protect people, and it’s a fight that we are grossly unprepared for” —Chris Musselman, formerly the National Security Council’s counterterrorism director under Trump

“I would expect them to have begun selected targeting through socially-engineered phishing activities focused on the oil and gas sector, the financial sector and the electric power grid in that order,” Stewart wrote. “There may be instances now where they already have some persistent access. If they do, I expect they would use it, or risk losing the access and employ that capability early in the escalation of the crisis.”

Recent reports indicate that Iranian cyberwarriors have stepped up their online operations, with a particular emphasis on preparing to attack US firms. Among other moves, they’re aiming to trick employees at major businesses to hand over passwords and other vital information, giving them greater access to a firm’s networks.

“When you combine this increase with past destructive attacks launched by Iranian-linked actors, we’re concerned enough about the potential for new destructive attacks to continue sounding the alarm,” Christopher Krebs, a top cybersecurity official at the Department of Homeland Security, told Foreign Policy last July.

All of this — proxies striking around the world, cyberattacks on enterprise — would happen while Iran continued to resist conventional American forces.

In the Strait of Hormuz, for instance, Iranian sailors could use speedboats to place bombs on oil tankers or place mines in the water to destroy US warships. The Islamic Republic’s submarines would also play a huge part in trying to sink an American vessel. And the nation’s anti-ship missiles and drones could prove constant and deadly nuisances.

Should US troops try to enter Iranian territory on land, Iranian ground forces would also push back on them fiercely using insurgent-like tactics while the US painfully marches toward Tehran.

Put together, Brewer notes succinctly, a US-Iran war would be “a nasty, brutal fight.”

Aftermath: “The worst-case scenarios here are quite serious”

Imagine, as we already have, that the earlier stages of strife escalate to a major war. That’s already bad enough. But assume for a moment not only that the fighting takes place, but that the US does the unlikely and near impossible: It invades and overthrows the Iranian regime (which Trump’s former National Security Adviser John Bolton, at least, has openly called for in the past).

If that happens, it’s worth keeping two things in mind.

First, experts say upward of a million people — troops from both sides as well as Iranian men, women, and children, and American diplomats and contractors — likely will have died by that point. Cities will burn and smolder. Those who survived the conflict will mainly live in a state of economic devastation for years and some, perhaps, will pick up arms and form insurgent groups to fight the invading US force.

Second, power abhors a vacuum. With no entrenched regime in place, multiple authority figures from Iran’s clerical and military circles, among others, will jockey for control. Those sides could split into violent factions, initiating a civil war that would bring more carnage to the country. Millions more refugees might flock out of the country, overwhelming already taxed nations nearby, and ungoverned pockets will give terrorist groups new safe havens from which to operate.

Iran would be on the verge of being a failed state, if it wasn’t already by that point, and the US would be the main reason why. To turn the tide, America may feel compelled to help rebuild the country at the cost of billions of dollars, years of effort, and likely more dead. It could also choose to withdraw, leaving behind a gaping wound in the center of the Middle East.

In some ways, then, what comes after the war could be worse than the war itself. It should therefore not be lost on anyone: A US-Iran war would be a bloody hell during and after the fighting. It’s a good thing neither Trump nor Iran’s leadership currently wants a conflict. But if they change their minds, only carnage follows.

“The worst-case scenarios here are quite serious,” Hanna told me.