While other generals were feted at the end of the war, the German immigrant and his all-black Army corps were sent to Texas to evict the French who had occupied Mexico, which was considered one of the most difficult assignments. Weitzel remained in command of the District of Rio Grande until 1866, when he was mustered out of the volunteer service on March 1. He reverted to his regular Army rank, but was promoted to major of engineers later that year and to lieutenant colonel in 1882. In August 1866 he began designing an expanded canal around the Falls of the Ohio on the Indiana side.