A force of Northern Confederation militia under General Philip Lodge and a force of Indiana militia under General Harry Chapin had entered Mexico del Norte in March 1846. Lodge had vowed to reach the Norteño capital of Conyers by September, and his army advanced south through the summer of 1846. However, in late August, Lodge's army encountered a Mexican force led by a Cheyenne war band under Chief Running Deer at Arroyo de Dios. Running Deer won a great victory, killing 10,000 of Lodge's men before the North Americans withdrew from Arroyo de Dios. Sobel does not give any figures for the size of Lodge's army or what other casualties it suffered, or the size of the Mexican force and what casualties it suffered. It is implied that the North American losses were severe enough to require a complete withdrawal from Mexico del Norte for the rest of 1846.
Sobel's source for the Battle of Arroyo de Dios is General Sir Hugh Brandon's The Lodge Campaign of 1845-48 (London, 1882).