President Donald Trump has ordered airstrikes on Syria as punishment for its chemical weapons attack last week in the town of Douma, which killed at least 42 people — including a number of children.

Understanding Trump’s decision requires understanding the real nature of the country’s horrific civil war. At its heart, it is a conflict between a regime that represents a minority of its citizens and the majority who want it gone. But over time, it has spiraled into an immensely complicated international war, with some of America’s most significant enemies and closest partners on various different sides.

“It has been one of the hardest issues that I’ve faced as president,” President Barack Obama said in a December 2016 press conference. “Syria is the most complex, complicated issue I have ever had to deal with," then-CIA Director John Brennan said during a 2016 discussion at the Brookings Institution.

Getting involved in this war in any serious way is immensely complicated and risky, which is why the Obama administration largely avoided doing so, and why it never struck Assad directly. When Trump struck Syria last April, after another large chemical weapons attack, he tried to needle by punishing the Assad regime specifically for its use of chemical weapons without getting dragged into the broader conflict. But clearly, the limited bombing didn’t send the desired message — Assad wasn’t deterred from using chemical weapons again.

So will Trump’s new bombing campaign be something similar, but will involve hoping for different results? Or will he try a bigger intervention, will all of the risks that entail? As of right now, we simply can’t be sure.

What we can do, however, is get at the context behind this decision: how the civil war in Syria began, how it evolved, when US foes like Russia got involved, and what two American presidents have been willing to do so far. What follows is a guide to these key issues, a summary of the key issues and what they mean for the prospect of any bigger Trump intervention.

How Syria’s civil war began — and why America cares so much

When Syrians rose up as part of the wave of Arab Spring protests against Middle East dictators in 2011, Assad quickly decided to emulate his father, Hafez, and try to tamp down the protests through the use of force. The goal was to turn the broad-based protest movement from a political struggle — which Assad’s unpopular regime was bound to lose — into a military one, where his control of the army meant he might be able to kill his way to victory.

"It was very much a strategic decision that the regime made, to militarize the conflict right away," Glenn Robinson, an associate professor at the Naval Postgraduate School, told me in an interview. "I think in their mind, and correctly, if this becomes a political battle where populations matter, the regime probably only has support of a third of the country. ... If this becomes a political contestation, the opposition has the numbers."

The grim way of implementing this plan, slaughtering protestors en masse until they were forced to pick up arms in self-defense, got Assad the war he wanted. In July 2011, defectors from Assad’s regime formed an organized militia called the Free Syrian Army to protect protesters and strike back at Assad. By January 2012, the Syrian uprising had devolved into a full-blown civil war pitting the FSA and other assorted rebel groups against Assad and his supporters.

The strategic stakes of this war, both for the Middle East as a region and for the United States, are enormous — for three main reasons.

The first is Iran. Syria's alliance with Tehran dates back to 1980 and is critical to Iran’s regional ambitions. It uses Syria to convey weapons and other goods to its proxy militias and allies, most notably Hezbollah in Lebanon and Hamas in Gaza. In return, Assad's regime gets military and political assistance from Tehran.

Iranian leaders saw the revolt against Assad as a threat not just to him but also to them. In 2012, Iran responded by sending in Hezbollah to fight on Assad's side, an intervention that played an important role in Assad’s campaign against the rebels.

At the same time, though, Iran’s backing made Assad into a target for some of America’s closest partners in the region. Since the Iraq War, and maybe earlier, the oil-rich Sunni Arab states along the Persian Gulf — particularly Saudi Arabia, the largest and strongest — had been embroiled in a sort of cold war with Iran, a Shiite theocracy. Both sides wish to steer the political course of the Middle East, and see the other as a fundamental threat to their security.

So when Assad began to teeter, the Gulf states saw an opportunity to unseat one of Iran's principal allies and started sending arms to the Syrian rebels. In March 2013, the Arab League voted to give its members explicit permission to arm the Syrian opposition. In May of that year, the Financial Times reported that Qatar alone had given $3 billion in aid to the rebels. In just one shipment in 2015, Saudi Arabia provided rebels with 500 deadly TOW anti-tank missiles.

The Syrian conflict thus became more than just a civil war: It became a proxy fight between Iran and America’s Gulf allies, whose outcome was of vital significance for both sides.

It also quickly became a proxy standoff between Washington and Moscow.

Russia’s ties to Syria go all the way back to the Cold War: According to one scholar, the Soviets "essentially built" the modern Syrian military in the 1960s. Continued support for the Assad government gave the USSR its most reliable ally and proxy in the Middle East.

Today, Syria remains one of Russia's few reliable allies outside of the former Soviet republics, a vestige of Moscow's former superpower status and a final military toehold in the Middle East. Russia maintains a valuable naval base today at Tartus, on Syria's Mediterranean coast, and has sold a number of surplus weapons to Assad. Between 2006 and 2010, 48 percent of Syrian arms imports came from Russia. Assad’s defeat, then, would have been a major strategic blow to Vladimir Putin’s regime — which has ambitions of restoring Russia’s Cold War status as a great power with global influence.

“This matters to [the Russians] ... more than it would if they had another 10 [allies to spare],” says Doug Ollivant, who oversaw Iraq policy at the National Security Council from 2008 to 2009 and is now a managing partner at Mantid International.

As a result, Russia has been backing Assad since the war began. In 2011 alone, Russia vetoed a UN Security Council resolution condemning Assad’s crackdown on protesters, and sold nearly $1 billion in arms to Assad’s regime.

Putin’s support for Assad has steadily escalated, with Moscow selling Assad advanced air defense systems and refurbished MI-25 attack helicopters. By September 2015, Russia was a direct participant in the war. Russian warplanes and helicopters are hitting Assad’s enemies from the air, and Moscow has artillery pieces bombarding rebels and special forces embedded with Syrian troops. Some estimates place the number of Russian soldiers in Syria today at around 10,000.

The third and final issue is terrorism. Since practically the beginning of the conflict, al-Qaeda had been sending forces into Syria, seeing a chaotic civil war as a great environment for them to use as a safe haven and a place to get recruits. By mid-2012, the Syrian al-Qaeda franchise, Jabhat al-Nusra, had allied with some relatively moderate rebels — and established themselves as one of the most effective anti-Assad fighting forces. Qatar in particular showered Nusra with cash in an effort to quickly topple Assad.

Put those three things together — Iran, Russia, and terrorism — and you get a sense of why both Obama and Trump have to varying degrees felt the need to involve America in a brutal and seemingly intractable conflict.

The US policy under Obama: (mostly) non-intervention

Obama was president during the bulk of the Syrian civil war, and his legacy as commander in chief will be shaped in part by what he chose to do there — and what he chose to avoid.

One of those things was to refuse to endorse any kind of large-scale effort aimed at toppling Assad — repeatedly overruling members of his administration, like Secretary of State Hillary Clinton and CIA Director David Petraeus, who wanted to provide more advanced weaponry to the rebels. Parallel CIA and Pentagon efforts to recruit, train, and arm Syrian rebels did little to shape the course of the conflict; in 2015, the Defense Department had to concede that its $250 million effort had trained a grand total of 60 fighters (that’s more than $4 million per trainee).

Perhaps the defining moment came on August 21, 2013, when Assad's forces launched sarin gas — a horrifying and deadly chemical weapon — into the Damascus suburb of Ghouta, killing up to 1,423 people. Most of the dead were civilians.

Prior to the attack, President Obama had declared chemical weapons use to be a "red line": If Assad used them, it would trigger an American military response. But after Ghouta, Obama didn't seem to want to follow through. He submitted a plan for punitive airstrikes in Syria to Congress, where lawmakers from both parties signaled that it was likely to fail.

Russia offered Obama a way out of his self-made dilemma. It brokered a deal with Assad where he would agree to give up his chemical weapons and submit to international inspections if the United States agreed not to attack Assad.

The Obama team accepted this as a lifeline. As they saw it, the critical issue was never the Syrian civil war, which the president had decided was too risky to intervene in. Rather, it was the use of chemical weapons — a particularly heinous act prohibited by international law. If the threat of American military force got Assad to back down from chemical use, that would make chemical weapons use less likely in Syria and in other conflicts without putting American troops in harm’s way or risking a wider conflagration.

“I’m very proud of this moment,” Obama told the Atlantic’s Jeffrey Goldberg. ““The overwhelming weight of conventional wisdom and the machinery of our national-security apparatus had gone fairly far. The perception was that my credibility was at stake, that America’s credibility was at stake. And so for me to press the pause button at that moment, I knew, would cost me politically [but] I believe ultimately it was the right decision to make.”

Critics, by contrast, saw it as a cop-out. They argued that if Obama wouldn’t intervene after drawing a “red line” about chemical use, Assad and his allies would see it as proof that the US would never intervene for any reason. This, according to some experts, is the key reason Russia decided to escalate so dramatically in 2015: They came to believe that Obama wasn’t as invested in trying to push out Assad as they were in trying to keep him in power, which means the US would largely give them a free hand. In essence, Obama’s attempt to deter chemical weapons use destroyed his ability to deter Russian intervention.

“It is precisely when Obama went for the chemical weapons deal over strikes in 2013 that Moscow understood it had escalation dominance in Syria,” Emile Hokayem, a Syria expert at the International Institute for Strategic Studies, wrote in April.

This is a debatable point, to be sure. But there is no question that Obama left office with a civil war that had already killed at least 470,000 Syrians grinding along with no end in sight.

Wait, wasn’t the US bombing Syria before Trump?

There was one major exception to Obama’s hands-off policy in Syria: the war on ISIS. But it was carefully limited, in ways that show why Trump’s strike was such a major shift.

After the militant group swept across northern Iraq in June 2014, and came to control a swath of territory in Syria and Iraq roughly the size of Great Britain, the scale of the terrorist threat became impossible to deny. This, together with the videotaped beheadings of two American journalists, prompted Obama to declare a plan to "degrade and ultimately destroy" ISIS on September 10.

The linchpin of the plan was a series of American airstrikes, both in Syria and in Iraq, supporting forces on the ground that were fighting ISIS. In Iraq, that meant the official Iraqi army as well as tribal leaders and Shia militias. But it wasn’t clear, initially, who that would be in Syria.

Most rebel groups were preoccupied fighting Assad, and had no ability to really refocus on the Islamic State. The same was true, in reverse, for Assad; he had long maintained a sort of de facto ceasefire with ISIS so he could focus on fighting the moderate rebels whom he saw as a bigger threat.

The US ended up settling on fighters from the Kurdish ethnic group, based in northern Syria near the Turkish border, as their key allies. These Kurds were mostly uninvolved in the main civil war, as their chief objective was carving out a Kurdish state in majority-Kurdish areas rather than toppling Assad’s regime in Damascus. Moreover, ISIS had invaded their territory, and was besieging a Kurdish city named Kobane at the time of the US intervention.

So the thousands of missions flown by American warplanes, and hundreds of US special forces deployed to Syria, were supposed to accomplish two things: cut ISIS’s supply lines between Syria and Iraq, and back Kurdish forces in their fight against ISIS.

On this metric, they’ve more or less succeeded. American airstrikes helped break the siege of Kobane and allowed Kurdish forces to launch a counteroffensive that swept over ISIS’s holdings in north central Syria. Today, a joint Kurdish-Arab military group called the Syrian Democratic Forces is camped out within miles of ISIS’s capital city, Raqqa — and are preparing to attack the city itself, with major backing from US troops.

But note the delicacy of this strategy when it came to the main conflict in Syria.

The United States was fanatical about limiting the scope of this counter-ISIS campaign — in particular, making sure it never became a counter-Assad campaign. When the Pentagon sent weapons to some Syrian rebels whom they wanted to fight ISIS, it made them promise not to use those weapons against Assad. (The tiny number of rebels who took the US up on this weak offer were swiftly slaughtered by al-Qaeda forces.)

This only intensified after the Russian intervention began in September 2015. US and Russian forces developed a procedure for flying in the same airspace, called deconfliction, designed to ensure that there was no accidental US bombing of Russians or Assad forces. US and Russian planes managed to fly near each other, for well over a year, with no major incident — even as Russian planes were on their way to drop bombs on Syrian civilians.

This made crystal clear to both the Russians and the Syrians that the United States had no interest in intervening in their war on the rebels. They developed a sense of impunity — enabling atrocities like Saturday’s chemical attack.

Trump has already escalated once. Will he go further this time?

There was little reason to believe that Trump would treat Assad and Russia more harshly than Obama did. During the red line debate in 2013, he tweeted repeatedly and angrily against a strike, warning Obama to back down.

“AGAIN, TO OUR VERY FOOLISH LEADER, DO NOT ATTACK SYRIA,” he tweeted at the time (all caps his). “IF YOU DO MANY VERY BAD THINGS WILL HAPPEN & FROM THAT FIGHT THE U.S. GETS NOTHING!”

During the presidential campaign, Trump even suggested that Assad’s rule was better for Syria than the alternatives. “We don't know who the rebels are,” he said in an October presidential debate. “If they ever did overthrow Assad ... you may very well end up with worse than Assad.”

Trump did not return to his “partner with Russia” policy after taking office. But he did shift the US’s Syria policy, at least rhetorically, with several administration officials suggesting the US would accept Assad staying in power. (Obama wasn’t doing anything substantive to oust Assad but was at least rhetorically committed to his departure.)

That all changed with last year’s chemical weapons attack. In April 2017, Assad used chemical weapons in the town of Khan Sheikhoun, killing more than 80 people — including children. At the time, Trump seemed genuinely angry and horrified by the photos and videos of children killed in the chemical weapons strike, and wanted something done about it.

This isn’t as much of a surprise as it might seem: Trump has always been more of an instinctive hawk than was genuinely believed. Back in March 2011, when Libya’s Muammar Qaddafi was killing his own people in the early stages of the country’s civil war, Trump released a video calling on then-President Obama to “stop this guy” and “save these lives.”

Thomas Wright, a scholar at the Brookings Institution who has spent the past several months studying Trump’s foreign policy doctrine, thinks a similar kind of emotional reaction might be at work here.

“There [have been] very few iconic images of brutality from the Syrian civil war,” he told me. “A picture of a lot of children gassed is pretty horrific for anyone to see.”

But the nature of the attack itself also mattered.

The goal of the 2013 deal Obama made with Syria wasn’t just to eliminate Syria’s chemical weapons stockpile (a goal at which it clearly failed). It was to show that chemical weapons were somehow different from normal weapons, more unacceptable. The agreement was supposed to show that rogue actors like Assad have two choices: either agree to stop using chemical weapons or face some kind of punishment.

Between 2013 and 2017, this had mostly worked: Assad had not used a nerve agent prohibited by the Chemical Weapons Convention (though he has used chlorine gas, a chemical weapon that isn’t explicitly banned).

What’s known as the “norm” against chemical weapons use — the idea that chemical weapons were, like nuclear and biological weapons, a special kind of evil to be prohibited even in vicious wars — had been upheld, seemingly backstopped by US threat of force.

Last year’s attack made clear that Assad was prepared to break that norm at will, something certainly not lost on Defense Secretary Mattis or then-National Security Adviser H.R. McMaster. They know how serious a violation of the norms around chemical weapons this attack is and how important it is to limit the use of such of weapons. They have likely told the president this, and informed him that American credibility is on the line.

“This is a kind of flagrant flouting of that agreement. ... It’s physical proof that they’ve duped the international community,” Michael Hanna, a senior fellow at the Century Foundation who follows the Syria conflict, says. “It is different than other kinds of war crimes committed by the regime, and ... that view is clearly going to be relayed to him by his close advisers.”

This appears to be the main reason Trump struck Syria, both last year and this time around. His goal wasn’t to intervene in any of the broader and more complicated dynamics of the war — to stop the civil war or weaken Iran. Rather, it’s designed to tell Assad that chemical weapon use will be punished. The goal of last year’s strike was, as Trump said explicitly, to “deter” future chemical weapons use by Assad. Clearly, deterrence failed. If Trump wants to establish it, he’ll need to make this volley more effectual than the last.

Meanwhile, the rebels are in trouble. Since the beginning of the Russian intervention in September 2015, Assad has steadily retaken territory; major rebel strongholds, like the city of Aleppo, have fallen to regime forces. “Assad won his war to stay in power,” Mara Karlin, a scholar at the Brookings Institution, concluded in February congressional testimony.

Another limited strike won’t change that dynamic. Last year’s attack did such little damage, in fact, that Syrian warplanes were already flying missions out of the sole airbase Trump targeted within a day of the airstrike. Trump would need to do something bigger if he wanted to really scare Assad.

There are two main questions going forward. First, what will the new strikes look like? Second, if they do turn out to be much bigger, could they escalate into something far larger, with Trump potentially being pressured into trying to not just constrain Assad but turn the tide of the conflict itself?

We don’t know the answers to these questions yet. But they will play a significant role in determining America’s role in Syria — and, possibly, the future course of the Syrian civil war itself.