Why have states stopped issuing declarations of war? Declaring war was a norm of international politics for millennia, but now appears to have exited states' behavioral repertoires. I argue that the proliferation of codified jus in bello, the law of war governing belligerent conduct, has created disincentives for states to issue formal declarations of war. The increasing number of codified international laws that govern belligerent conduct during warfare has made complying with the laws of war extremely costly. One way for states to limit these costs is to avoid admitting they are in a formal state of war by refraining from declaring war. I test this claim, as well as others, using an original data set. I also discuss several cases of nineteenth and twentieth century wars that illustrate the logic of this argument.