Here are seven key points for understanding this dramatic shift in policy:

Why would Assad use chemical weapons in the first place? Didn't he know that he would provoke the United States to intervene?

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In short: He's running out of options.

As Andrew J. Tabler of the Washington Institute for Near East Policy helpfully explained to me, Assad has just 18,000 deployable forces and his government controls only a third of the country. That means his army is stretched very thin. Additionally, he's being pressured by the Russians to go on the offensive. They especially want him to press eastward, so that if/when Raqqa falls out of the hands of the Islamic State, his army will be there to pick up the pieces.

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"He's stretched his forces too thin," Tabler told me. "And he's begun losing ground" in the area where the sarin attack was perpetrated Tuesday. Assad doesn't have the forces he needs to retake that region, so he turned to his stash of chemical weapons. And although chemical weapons have provoked a strong international outcry in the past, Assad probably thought he could get away with it.

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He probably thought that, Tabler says, because that's what Putin told him. As Tabler explains, U.S. officials say there were Russian soldiers at the air base from which the sarin attack was allegedly launched. "I think Assad and the Russians thought they had carte blanche," Tabler says. "They were wrong. "

Why did the United States decide to attack after staying out of the conflict for six years?

It's hard to puzzle out exactly what changed for the Trump administration, which has disavowed military intervention in the Middle East on several occasions. Just this week, Tillerson told reporters that removing Assad was not a priority. He was echoing comments made by United Nations Ambassador Nikki Haley and even White House press secretary Sean Spicer. They made these statements knowing that Assad had used chemical weapons at least three times since 2013, when a sarin gas attack killed more than 1,000 people.

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But if you listen to the president, it had everything to do with the pictures. On Tuesday, 80 people were killed by a sarin attack perpetrated by the Assad government. (Assad's government disputes that charge.) On Wednesday, Trump told reporters that he had been deeply affected by the pictures coming out of Idlib province. The images showed, in Trump's words, "beautiful little babies" lying dead on the ground, suffocated by a chemical agent. Trump called the attack an "affront to humanity." By Thursday, he was saying that "something should happen" to Assad because of his responsibility for the attack.

Why did the United States target Syria's Shayrat air base? And what did the attacks really accomplish?

Thursday night, the U.S. military launched 59 cruise missiles at a Syrian military airfield. Why did they choose the airfield, and not, say, one of the places where Assad's chemical weapons are stored? That's pretty easy: Shayrat, in Homs province, is the base that apparently launched the chemical attack in Idlib. The U.S. missiles targeted air defenses, aircraft, hangars and fuel-storage sites. The U.S. military said initial indications were that the strike had “severely damaged or destroyed Syrian aircraft and support infrastructure," though that's hard to independently verify.

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According to Reuters, the base is still at least partially functional.

Syrian officials say the strikes also killed seven people, including four children. Nine others were wounded, according to state media reports. As one local told the BBC, the blasts from the strike were "massive." He thinks civilians living nearby could well have been killed.

Iran and Russia condemned the attack. What do they stand to lose?

The war in Syria is a civil war, but it's also a proxy war, being fought by many, many, many other countries. And the places with the most to lose if Assad falls? Russia and Iran.

Moscow is Assad's key international ally, and has provided the Syrian leader with unconditional diplomatic backing. Since September, Russia has provided direct military support as well. Experts say it's conducted more than 500 airstrikes against rebel-held forces. Russia's motivations are myriad — involvement in Syria makes the country a major player in the Middle East, and protects Russia's only Mediterranean military base. It also gives Russia a way to play international peacekeeper.

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Iran, too, has more than 2,000 troops bolstering the Syrian army, but Tehran publicly denies being involved. Iran's motivation for helping Assad are also wrapped up in its regional ambitions. Iran is the Middle East's most powerful Shiite state, and it looks to bolster other Shiite regimes, like Assad's government. Iran is also deeply concerned about the rise of the Islamic State and al-Qaeda, Sunni terrorist groups that have targeted Shiite Muslims in the past. "If Syria falls, Iran will have to fight these groups within its borders," an expert told Al Jazeera in 2016.

Why didn't Obama do something similar after Assad used sarin on civilians in 2013?

In 2013, President Barack Obama was confronted with a similar choice. A sarin attack on the outskirts of Damascus killed more than 1,000 people, including 400 children. The Assad government was allegedly to blame. And Obama had already drawn a "red line," promising to attack if chemical weapons were deployed.

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After the Ghouta assault, the president considered a targeted military response. But Congress was unwilling to authorize the strike. So instead, the president sought out diplomatic alternatives. With an assist from Russia, he made progress — at the end of 2013, Assad agreed to destroy his country's massive stockpile of chemical weapons and also agreed to United Nations monitoring. By June 2014, he had largely complied.

In truth, though, Obama was always deeply reluctant to intervene in Syria, concerned about getting the U.S. entangled in another intractable war.

As the Atlantic's Jeffrey Goldberg explained:

In 2013, Obama feared, not without justification, the second- and third-order consequences of an American missile strike on the regime. Even before he became president, Obama worried greatly about slippery slopes in the Middle East. In Syria, he understood that Assad would most likely survive an American missile strike on his airbases; the day after such strikes ended, Assad, Obama believed, would have emerged from his hiding place, and declared victory: The greatest power in the world tried to destroy him, and failed. Obama was acutely aware that a one-off strike (a theoretical strike described as 'unbelievably small' by his secretary of state, John Kerry), could possibly have served as a convincing brush-back pitch, but he was also aware that such a limited strike could have been wholly ineffectual, and even counterproductive. Assad and his allies, understanding that the appetite of average Americans for yet another Middle Eastern war was limited, could have tried to provoke Obama into escalation. An all-out war against the Syrian regime would have been, in many ways, Obama’s Iraq. And Obama wasn’t interested in having his own Iraq.

Even in 2016, Obama told Goldberg that he was "very proud of this moment."

Aren't U.S. forces already already bombing targets in Syria?

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Yup. In 2014, the United States began a military campaign against the Islamic State in Syria. U.S. planes have bombed hundreds of targets over the past two years. There are also hundreds of U.S. troops on the ground near Raqqa, helping rebel groups prepare an offensive on the city, which is controlled by the Islamic State.

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Some experts are warning that Trump's attacks will make America's assault on the Islamic State harder. As Andrew Exum wrote in the Atlantic:

For the past two years, U.S. and coalition aircraft have flown in and around one of the world’s more robust air defense systems without the Syrian regime harassing the pilots. We had a few incidents where Russian jets got too close to U.S. aircraft or Syrian anti-aircraft radar lit up U.S. or coalition aircraft, but for the most part, the air war has gone forward unimpeded. Both Russia and the Syrian regime, though, are still well-positioned to play the spoiler. They can affect the flights of U.S. aircraft in eastern Syria by activating their air defenses and have, in recent months, brought in more advanced air-defense weaponry that has even the Israelis nervous. They’ve also “accidentally” struck U.S.-backed rebel groups fighting the Islamic State.

Putin spokesman Dmitri S. Peskov said the strikes represent a "significant blow" to American-Russian ties. Friday morning, Russia said that it was suspending its pact with Washington to share information about warplane missions over Syria. The pact had protected U.S.-led coalition forces waging airstrikes against Islamic State targets within the country.

Is this attack a big deal? Could it escalate into some kind of horrible America/Russia World War III situation?

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Honestly, probably not.

Last night's strike does not change the reality on the ground in Syria. It does not greatly impact Assad's military capabilities. It won't lead to any kind of regime change, at least not without a significantly broader U.S. military intervention. It probably also won't hurt America's (admittedly frosty) relationship with Russia in the long term. Sure, Russian officials blustered this morning. But behind the scenes, the country seemed willing to give the administration a pass, particularly because the U.S. made sure no Russians were hurt.

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What's scary — really, truly scary — is what this says about Trump's temperament. Earlier this week, Tillerson, Haley and Spicer all offered some variation on a theme: Assad would, and should, stay in power. Ousting him was not a U.S. priority. That seemed to jibe with Trump's own statements during the campaign, when he called the Iraq war stupid and promised not to get entangled in the Middle East. (It also contradicts Trumps 2013 position. After the sarin attack in Ghotta, Trump wrote: AGAIN, TO OUR VERY FOOLISH LEADER, DO NOT ATTACK SYRIA - IF YOU DO MANY VERY BAD THINGS WILL HAPPEN & FROM THAT FIGHT THE U.S. GETS NOTHING! Here are 17 other times he's made a similar claim.)

Now, just four days later, the Trump administration has done a 180.

What's changed? The president saw some really upsetting pictures on television, and felt moved to act.

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Having a change of heart is not the same thing as having a plan, however. Syria's civil war is one of the most complicated foreign policy problems in the world. A dozen countries are now embroiled in the outcome. And if Assad goes, it's not clear what comes next. Best-case scenario: Syria is Iraq. Worst case, it's Yemen: violent, fractured, and fertile ground for terrorists.

There's no evidence that Trump has grappled, in the past few days, with what the Syrian people need, and what the United States can and should invest. At a news conference Thursday, he offered this assessment: "I think what happened in Syria is a disgrace to humanity. He's there and I guess he's running things, so something should happen."