It took less than a week for the Trump administration to completely reverse its policy on Syrian President Bashar al-Assad. On Friday, the White House said that Assad’s continued leadership of the war-torn country was “political reality.” Secretary of State Rex Tillerson said, “I think the status and the longer-term status of President Assad will be decided by the Syrian people.” UN Ambassador Nikki Haley said U.S. policy is “no longer to sit there and focus on getting Assad out.”

That was then. Suddenly, after a chemical-weapons attack earlier this week, the U.S hasn’t just done a 180-degree rhetorical turn; it’s now talking about military options to remove Assad. On Thursday, Tillerson said “steps are underway” to remove him from power. CNN’s Dana Bash reported that President Trump has told members of Congress he’s considering military options. Reuters also reported Trump and Defense Secretary James Mattis “are in detailed discussions on military options to respond to a poison gas attack in Syria that killed scores of civilians,” with Mattis headed to Trump’s Mar-a-Lago resort in Florida for the weekend.

Speaking to pool reporters on board Air Force One, the president offered a vague statement of his own about Assad. “He’s there, and I guess he’s running things, so I guess something should happen,” Trump said, sounding as though he were sliding toward putting U.S. troops into action without really understanding  why or wanting to do it.

It’s a breathtaking change of course, from the non-intervention of Trump’s campaign and early presidency to a sudden, noisy presentation of the war drums. Other presidents have campaigned on peace and made war; this week marks the centennial of Woodrow Wilson taking the U.S. into World War I, and George W. Bush campaigned against nation-building before he embarked on two of the costliest nation-building experiments in American history. No one yet knows what sort of course Trump might take, in part because he continues to insist he doesn’t want to tip his hand, even though all indications are he hasn’t even figured out what cards he holds yet.

Were Trump to opt for military action, it would be faster and with less incitement than either of those predecessors. As horrific as Tuesday’s chemical attack is, it’s similar to previous actions by the Assad regime, which Trump shrugged off as not America’s problem. But any attempt at military intervention would face a range of obstacles, both legal and practical.

One is finding a legal justification for a strike. The U.S. is already flying missions over Syria and Iraq to combat ISIS, but the Authorization for Use of Military Force (AUMF) under which they are doing so would not cover an attack on the Assad regime. (Many legal analysts do not believe the existing AUMF even justifies the attacks on ISIS.) In 2013, after another chemical-weapons attack in Syria, President Obama considered military strikes against the Assad regime but ultimately decided he could not act without congressional authorization. Congress was in no hurry to grant it.

Some hawks accused Obama of using Congress as an excuse, and indeed previous presidents have launched attacks without seeking permission first. (In August 2013, however, Trump was sure that Congress must authorize any strike.) In practice, there’s usually very little Congress can do to head off such an attack before it happens. If it’s popular, Congress will generally go along; if it’s not, public opinion brings it to an end.

Even then, however, presidents have sought some sort of justification under international law. When Ronald Reagan bombed Libya in 1986 and when Bill Clinton bombed Afghanistan and Sudan in 1998, they each argued the attack was justified under Article 51 of the United Nations Charter, which allows nations to act in self-defense. It’s difficult to imagine how attacking Syria would suddenly and newly qualify as self-defense, especially given Trump’s blasé attitude toward Assad before Wednesday. (Jack Goldsmith, a former Bush administration official, made the case that action in Syria would be illegal in detail in 2013.)  Tillerson suggested Thursday the U.S. might justify an attack on the grounds that Assad’s use of chemical weapons violates past UN resolutions, saying it is “a serious matter. It requires a serious response.”

The international backlash might not be limited to complaint about rule-breaking; there are serious practical matters to consider, too. For one, whence would the U.S. launch any attacks? The Washington Post reports:

The United States has a broad arsenal already in the region should Trump decide to attack, 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 gunship helicopters. 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.

Nations hosting U.S. planes attacking ISIS might not be so eager to be a base for attacks on Assad, but Turkish President Recep Tayyip Erdogan has already said he would support U.S. action, which might answer one question.

Once U.S. aircraft were headed into Syria, they would have to negotiate the problem of Russian troops already there. Though the Kremlin said Thursday its support for Assad is not unconditional, there are Russian forces currently on the ground, along with sophisticated anti-aircraft systems. If American attacks killed Russians, it could spark a much larger diplomatic or military encounter. (The presence of Russian forces in Syria was one reasons some experts dismissed Hillary Clinton’s proposal for a no-fly zone in the country.)

All of this assumes Trump would begin with a barrage of airstrikes. (The U.S. does have limited ground troops already in Syria to fight ISIS, too.) One question is whether the president would pursue a quick strike and then attempt to extricate himself, or try something more sweeping. When Obama was toying with action, he very publicly pursued a limited strategy. Secretary of State John Kerry was widely mocked for saying any attack would be “unbelievably small,” but Kerry’s attempts at reassurance pointed to the central dilemma: What could a highly limited attack really achieve? Obama concluded the answer was not enough. If Trump decided for a small effort, what would the objective be? To kill Assad? And if it didn’t work immediately, what would he do next? Expand his fight? The risk of getting drawn into a protracted war is no less than it was in 2013.

If, on the other hand, Trump sought a more extensive strategy, all of the logistical, legal, and political questions would be even more complicated. Given how new the Trump administration is, and how understaffed it remains, the president can hardly have had time to consider the sorts of planning required for a major bombing campaign, much less a larger war. When Obama joined the intervention in Libya, it came a month into the Libyan civil war and was coordinated with other nations. Even so, that action turned into a catastrophe. The Iraq war was the product of months of planning, and its aftermath was even worse—bloodier and more costly. The Trump administration is giving every indication it’s on a much faster path than either of these.

These worries should be at the front of Trump’s mind in an especially concrete and emotional way, since he spent Thursday morning participating in the Wounded Warrior Project Soldier ride, a tribute to members of the armed services injured during the quagmires in Iraq and Afghanistan. Trump faces the same problem that Obama did on Assad: The Syrian leader is a bloodthirsty tyrant, and the U.S. possesses vast military might. But there are still no good answers on what to do with that military might, how to do it, or what comes afterwards.