Historically, crystallization of new rules of customary international law was viewed as a protracted process that took decades, if not centuries, to complete. But, sometimes, during periods of sweeping geopolitical change, customary international law can ripen quite rapidly. Often those periods correspond with major wars. Named in honor of Hugo Grotius, whose masterpiece De Jure Belli ac Pacis helped usher in the modern system of international law at the end of the Eighty Years’ War, “Grotian moments” are transformative developments that generate the unique conditions for accelerated formation of customary international law. Has this book proved that the Syrian conflict was a Grotian moment? And, if so, what will the legacy of Syria be?