The move represents a remarkable turnaround for Trump, who had been unequivocal about who he thought the enemy was in Syria—ISIS—and said during the presidential campaign he believed fighting Assad and ISIS at the same time was both “madness and idiocy.” And yet the U.S. finds itself doing just that: The U.S. military has struck ISIS in Syria since 2014, and its troops are at present working with local fighters to retake Raqqa, ISIS’s de-facto capital. As recently as this week, Secretary of State Rex Tillerson and Nikki Haley, the U.S. ambassador to the UN, said removing Assad was not a U.S. priority. But after Tuesday’s attack, Trump said the images from Khan Sheikhoun, the target of the chemical-weapons attack, “changed his mind” about Syria.

The Washington Post reported that Trump has several military options in the region:

The United States has a broad arsenal already in the region … including dozens of strike aircraft on the USS George H.W. Bush, an aircraft carrier that is deployed to the Middle East and accompanied by guided-missile destroyers and cruisers that can also launch Tomahawk cruise missiles. Additionally, an amphibious naval force in the region that includes the 24th Marine Expeditionary Unit could muster Harrier jets and Cobra gunships. The Pentagon also has scores of aircraft in the region flying operations every day against the Islamic State group, including from Incirlik air base to the north in Turkey.

The strikes deep inside Syria represent a major shift in U.S. policy. The Obama administration opted not to strike Assad or oust him using force despite his use of chemical weapons and crossing President Obama’s metaphorical “red line” in 2013 on the use of the such weapons. The U.S. focused instead on arming moderate rebels—an exercise that ultimately proved an embarrassing failure—and working with Russia on an agreement to destroy Syria’s chemical-weapons stockpile, an agreement that given this week’s attack appears not to have worked.

The U.S. strikes Friday risk exacerbating a delicate balance in Syria. Assad now controls more of the country than at any point since the civil war began there more than six years ago. He is there in large part because of his patrons in Russia and Iran, whose militaries are fighting in Syria against opposition groups; his position has also been strengthened by U.S. and allied strikes against ISIS, one of the most effective anti-Assad groups. The Syrian civil war has killed hundreds of thousands of civilians and created a refugee crisis whose effects are being felt far beyond Syria and the region. A cease-fire brokered last December by Russia, Iran, and Turkey between Assad’s forces and most rebel groups (ISIS and al-Qaeda are excluded) is mostly holding, and it’s unclear whether the latest developments will change that. Also unclear is what Russia will do. It has adamantly opposed Western military intervention in Syria, arguing that whoever comes after Assad will be worse. As my colleague David Graham wrote on Thursday: “If American attacks killed Russians, it could spark a much larger diplomatic or military encounter.” Davis, the Pentagon spokesman, cited a U.S. official as saying Russians were advised of the impending U.S. attack.