An “optional war” is a war that a state is permitted but not required to wage. Are there any such wars? This article assesses the two most promising arguments for optional war. (1) Permissible humanitarian wars can be so costly for soldiers and taxpayers that states are not be required them. (2) Wars of national self-defense can be discretionary: states can sometimes choose whether or not to defend themselves. The article refutes both arguments. Pace (1), states should not wage wars (...) when the costs of doing so are overly burdensome. Pace (2), states are required to resist aggression whenever they are morally permitted to do so. (shrink)