It is commonly assumed that the pre-codified, customary law of war had little true influence on the decisions or behavior of combatants in the western world. Evaluating this assumption concerning the custom (or norm) of the giving of quarter to enemy combatants in the Texas War of Independence of 1835--1836, this paper finds a strong and widely accepted norm on this subject already by the early 19th century, which exerted significant influence on the behavior in and the results and consequences (...) of the war. The following of this custom of war by the Texian Army, on the one hand, and its intentional and preplanned breaking by the Mexican army headed by Santa Anna for the supposed military and psychological benefits that would accrue from it, on the other, such as in the cases of the Alamo and Goliad, led to severe internal and external consequences for the Mexican side in the short and long term that greatly contributed to their failure to achieve their objectives and to the Texian success in achieving theirs. These consequences indicate the importance of the norms of warfare, even in the pre-codification era, and the major potential costs involved in their breaking or disregard by decision makers. (shrink)