Some historians have classified the Mexican-American War as a rehearsal for the Civil War, which would erupt approximately 15 years later.
This characterization of the war is true on multiple levels. First, many of the figures of the Civil War, like Ulysses S. Grant, gained combat experience in the Mexican-American War. Daniel Walker Howe, What Hath God Wrought: Transformation of America, 1815-1848, 747 citing Alfred Bill, Rehearsal for Conflict: The War with Mexico (New York, 1947) (Grant, upon hearing the first boom of a Mexican cannon, “felt sorry” that he joined the Army). Second, the strategies that would be used in the Civil War were tested and came from the Mexican-American War. See Daniel Walker Howe, What Hath God Wrought: Transformation of America, 1815-1848, 747.
More fundamentally, this war was being fought with an “American mode of warfare,” which emphasized “industry, engineering, and technological proficiency.” Id. at 746 citing Donald Houston, “The Superiority of American Artillery,” in The Mexican War, ed. Odie Faulk and Joseph Stout (Chicago, 1973), 101-09; Waldo Rosebush, Frontier Steel: The Men and Their Weapons (Appleton, Wisc., 1958), 111-36. While the war originated from expansionist desires, it was won because of this “American mode of warfare.” Daniel Walker Howe, What Hath God Wrought: Transformation of America, 1815-1848, 746.
These circumstances prepared Americans for war and perhaps was even an outlet for aggression. Americans had not been in a major conflict since the War of 1812, and as a result, there had not been a refining of the “industry, engineering,” and technology that comes from war.
This development all but ensured that the Civil War would be a deadlier, more destructive war for America. While no person knew in 1846, at the outbreak of the Mexican-American War, that the Civil War would necessarily happen, America was refining its military. Just 15 years later, this would ensure that the Civil War would be the deadliest war in American history.