During the Mexican-American War and his service in it, Grant started to formulate his own beliefs and judgments about warfare. He judged the actions of high officers (though not out loud at the time, obviously) and decided that this war was immoral. He disagreed with actions taken by superiors, and believed the entire premise of the war to be a front for acquiring more land that was unopposed to slavery. In his best-selling memoirs written at the end of his life, Grant reflected on his experience in the war, saying, "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 also stated, "I do not think there was ever a more wicked war...I thought so at the time...only I had not moral courage enough to resign."
It may seem odd to us that a great general like Grant would be opposed to wars as "immoral" or "unjust." To him, however, he saw nothing in the Mexican-American war except a big bully provoking a puny child to respond, and beating him to a pulp (and taking his lunch money) when he did inevitably retaliate. His experience in this war helped him formulate his ideas, ethics, and beliefs that would characterize his command of the U.S. Civil War.