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.”