However, Grant also wrote that the Mexican war was morally unjust and that the territorial gains were designed to expand slavery, stating, "I was bitterly opposed to the measure ... and to this day, regard the war which resulted as one of the most unjust ever waged by a stronger against a weaker nation."He opined that the Civil War was divine punishment on the U.S. for its aggression against Mexico.[66] During the war, Grant discovered his "moral courage" and began to consider a career in the army.[67]
Historians increasingly have pointed to the importance of Grant's experience as an assistant quartermaster during the war. Although he was initially averse to the position, it prepared Grant in understanding military supply routes, transportation systems, and logistics, particularly with regard to "provisioning a large, mobile army operating in hostile territory," according to biographer Ronald White.[57] Grant came to recognize how wars could be won or lost by crucial factors that lay beyond the tactical battlefield. Serving as assistant quartermaster made Grant a complete soldier, and learning how to supply an entire army gave Grant the training to sustain large armies.