In the contemporary era, territorial conquest has been seen as illegitimate and has taken place in only limited ways. According to an influential narrative in scholarship and public debate, this “territorial integrity norm” is a product of the post-World War II international order and contrasts with the nineteenth century, when conquest was normalized and “might made right.” This essay argues, however, that nineteenth-century European international law imposed meaningful limitations on conquest, including “territorial inviolability.” These limitations were more effective in the colonized world than in Europe, primarily because national irredentism was not thought relevant outside Europe. Europeans’ denial of non-European sovereignty contrasted with their respect for European-established colonial boundaries, and they did not fight over colonial territory between 1815 and 1914. I demonstrate the strength of this “etiquette of thieves” by examining two events where territorial conflict between colonial powers was narrowly avoided: the Panjdeh (1885) and Fashoda (1898) incidents. Viewing territorial integrity as qualitatively changing, rather than absent at one time and present later, has important implications for discussions of how recent conquests, such as those of Russia in Ukraine, will affect the principle of territorial integrity. In particular, territorial integrity may be more likely to be altered in how it is applied than eroded altogether. A specific form of territorial integrity is an integral part of the post-World War II international order, but constraints on conquest as such need not be limited to that specific version of territorial integrity.