In 49 BCE, Julius Caesar traveled with his troops to the banks of the Rubicon river in northern Italy. By an ancient law, no Roman general was allowed to cross the river with an army. Caesar paused momentarily, weighing the terrifying prospect of civil war. Then, according to the Roman historian Suetonius, Caesar declared “the die is cast,” and swept south toward Rome. This week, President Trump struck a Syrian airfield with dozens of cruise missiles, following an attack by the Syrian regime on civilians with sarin nerve gas. Has Trump crossed his own Rubicon, and reached a point of no return, with the prospect of full-scale embroilment in the Syrian morass?

There’s good reason to think the air strikes will be an isolated event. After all, none of the key players in Syria have an interest in triggering a major conflict. Bashar al-Assad is winning the civil war. Why snatch defeat from the jaws of victory by taking on the U.S. military? Meanwhile, Trump has long pressed for getting out of wars in the Middle East, and focusing on nation-building at home. Similarly, Russia has no desire for a larger contest against the United States. Trump’s proposed increase in U.S. defense spending is about the same as Russia’s entire defense spending. Trump reportedly received three options for a military strike, and chose the most limited. All of this suggests that cooler heads may prevail.

But the history of recent U.S. military operations shows how missions can morph unexpectedly into larger endeavors. “Once on the tiger’s back,” warned Undersecretary of State George Ball about U.S. intervention in Vietnam, “we cannot be sure of picking the place to dismount.” America’s recent big wars—in Afghanistan and Iraq—were supposed to be speedy regime change operations, but both turned into costly and prolonged counter-insurgency campaigns. Smaller-scale peacekeeping and humanitarian operations, which might seem more analogous to the Syrian strikes, were also subject to mission creep. For example, the U.S. intervention in Lebanon in 1982 began as part of a multinational effort to oversee the withdrawal of Syrian and Palestine Liberation Organization (PLO) fighters from Beirut. This mission went smoothly, but the goals broadened and the United States became an active player in the Lebanese civil war—until a suicide bomber struck the marine barracks and killed 241 Americans. Similarly, the humanitarian mission in Somalia in 1992 started out as a limited operation to deliver food but shifted into a broader nation-building operation to stabilize war-torn Somalia—a road that led to the deaths of 18 Americans in the “Black Hawk Down” battle.

Of course, these interventions all involved U.S. ground troops from the start, so what about bombing campaigns? Again, the tiger’s back often proved a bumpy ride. In 1999, NATO launched air strikes to force Serbia to remove its troops from Kosovo. Serb leader Slobodan Milosevic responded by weaponizing refugees, terrorizing Kosovo’s civilians and sending them fleeing to the border in a bid to split the NATO coalition. The air war dragged on for months, and NATO was looking at the grim prospect of a ground invasion, when Milosevic suddenly caved. Perhaps the best analogy to the Syrian strikes is Libya in 2011, which began as an air campaign to shield civilians but quickly escalated into a war to overthrow Qaddafi. A British Parliament inquiry concluded that, “By the summer of 2011, the limited intervention to protect civilians had drifted into an opportunist policy of regime change.”

The use of force doesn’t automatically escalate. In 1998, al-Qaeda bombed two U.S. embassies in Africa, and Bill Clinton responded with cruise missile strikes against facilities in Afghanistan and Sudan. For the White House, it was fire and forget. But in another sense, the missile strikes represented the opening exchanges in a larger contest. Republican hawks believed that cruise missiles were so weak as to invite further terrorist attacks. George W. Bush said, “the antiseptic notion of launching a cruise missile into some guy’s, you know, tent, really is a joke. I mean people viewed that as the impotent America.” After 9/11, the Bush White House was offered the option of attacking Afghanistan solely with cruise missiles. Journalist Bob Woodward captured the response: “It might as well have been labeled the Clinton Option. There was palpable disgust at the mere mention of cruise missiles only.” Again, small strikes ultimately became a big war.

We’ve already seen signs of mission creep in Syria. This week, the Trump administration completely reversed its position on Assad’s political future. Having stated that removing Assad was no longer a priority, the administration then announced that there was no place for the dictator in Syria’s future, and “steps are underway” to remove him. After the air strikes, U.S. Ambassador to the United Nations Nikki Haley said the United States was “prepared to do more.” Russia announced that relations with the United States were “completely ruined.” Hawkish senators like John McCain and Lindsey Graham urged a broader mission to ground Assad’s entire air force.

Wars have a habit of evolving in unexpected ways due to a combination of psychology, domestic political pressures, and strategic interactions. Psychologists have found that the act of committing to a decision—like launching air strikes against Syria—can make decision-makers overconfident that they made the right choice. A classic study from the 1960s showed that after people made a bet at a racetrack they immediately became more confident their horse would win. Psychologists Heinz Heckhausen and Peter Gollwitzer later developed this idea into the “Rubicon Model of Action Phases,” named after Caesar’s fateful choice in 49 BCE. When people first weigh up different options, they are fairly objective and clear-headed. But after making a decision, they become zealots of their chosen course of action—and suddenly wildly optimistic it will succeed. As Gollwitzer put it: “Choosing between action goals leads to realism; and implementing chosen goals leads to positive illusions.” After Trump crossed the Rubicon, any doubts he had may have been replaced by confidence—the kind of mindset that could easily broaden the war.

Another escalatory dynamic comes from how the operation is framed. Leaders sell military campaigns to the American people using highly moralistic language. But this makes it difficult to end the war with anything less than regime change. Trump described the chemical weapons attacks as “a disgrace to humanity” and said, “no child of God should ever suffer such horror.” If Assad is so evil that the United States must use force, how can America let this devil stay in power?

And, of course, the other guy gets a vote, which can dramatically shape the course of the fighting. Assad, or one of his allies like Hezbollah or Iran, might conceivably retaliate against the United States or Israel, essentially forcing the White House into further escalation. More likely, however, is that Assad will respond by pushing boundaries and then claiming victory. Limited air strikes rarely cause regimes to significantly alter their behavior for one simple reason—the regime doesn’t want to look weak, either at home or abroad. Assad is engaged in an existential fight. His country is in ruins. Half the population is displaced. He’s not going to bend to America’s will because of a few craters on an airfield. Instead, Assad will probably respond by avoiding an explicit repeat of the sarin gas attack, and returning to tried and tested techniques like systematic torture, barrel bombs, chlorine weapons, or the real weapon of mass destruction, shelling—all the while boasting about his defiance of America.

This will create an acute dilemma for Trump, as the children of God suffer new (and old) horrors. The president may feel overwhelming pressure to act to protect America’s—and his own—image of toughness and credibility. Every day that Assad kills and murders without a response is another day that he called Trump’s bluff. Will the red line really remain fixed around using sarin nerve gas—leaving all other behaviors acceptable? It’s not hard to see how the campaign escalates over time, as Trump launches broader strikes against the Syrian military, and regime change becomes the explicit goal.

For Trump, the dice are in the air.