New defense technologies often challenge existing international laws and norms, raising complex questions about whether new weapons should be normalized or banned. This paper examines how great powers have sought to manage the adoption of new weapons and the consequences for weapons norms. It compares the introduction of submarines and gas to battle in World War I and the consequences for postwar regulations. In both cases, the weapons were condemned as barbaric and inhumane, even as belligerents sought to manipulate existing rules of war to justify or condemn their use. Yet, after the war, only attempts to ban poison gas succeeded, while the submarine has become an accepted defense technology. What explains this variation? I argue that differences in weapons rhetoric, use, and institutionalization during World War I had long-term and unexpected consequences for norm creation. Wartime rhetorical strategies and sustained postwar domestic campaigning about poison gas, in particular, inadvertently raised and prolonged public fears that were not sustained in the case of submarines. The paper also suggests lessons for current policy debates, as well as insights into the political processes behind the development of norms of war. The article can be viewed here.