I think it is difficult to point the finger at any one cause of an event of such importance as the Civil War. It is clear that the Mexian War was certainly part of the process that resulted in the Civil War, but we can definitely not say it was the only reason or indeed that the seeds for the Civil War were sown through this war. It is important to analyse the Mexican War in terms of an overall process leading to the Civil War, making sure we analyse it in context with other historical factors.
Militarily, the American army showed its might on foreign soil during the Mexican War, and many of the Civil War's most important leaders gained their first combat experience in Mexico. When it came time for the Union and Confederate armies to recruit, they quickly took advantage of the ranking officers in the U. S. Army. Additionally, men like "Stonewall" Jackson, a Mexican War hero who had turned to teaching, were quickly recognized as the perfect candidates for high command because of their previous experience in Mexico.
The actual seeds themselves were planted at the time of the Revolution and the Constitutional Convention, when the country began to split between legal slavery and states that abolished it, and a founding document was adopted which did almost nothing to address the issue for the future.
However, the Mexican-American War did accelerate the onset of Civil War, adding massive amounts of territory to the Union with no set manner to distinguish if they would be slave or free states. The decade following the Mexican-American War was one of the most contentious and divisive in US history, and the events of the 1850s hurtled the country inevitably towards political and military conflict.
I must voice my disagreement with Post #1; the Mexican War did indeed sow the seeds of the Civil War. The slavery issue had been a burning issue between the Northern and Southern states; however they had entered into an uneasy compromise until the Mexican War. The problem lay not so much with the war itself as with the new lands obtained during the war. John C. Calhoun called Mexico
the forbidden fruit; the penalty of eating it would be to subject our institutions to political death.
Ralph Waldo Emerson commented:
The United States may conquer Mexico, but it will be as the man swallows the arsenic.
Less than three months after the end of the war, the issue of slavery in the territories raised its ugly head. Congress had finished its work on August 8, 1846; when a freshman Congressman from Pennsylvania threw a monkey wrench in the works, and set off a hornet’s next. His name was David Wilmot. Wilmot delivered a provocative speech before the house, in which he said that if new territory were added to the union, "God forbid that we should be the means of planting this institution [slavery] upon it." Wilmot used the words of the Northwest Ordinance and proposed that "neither slavery nor involuntary servitude shall ever exist in any part of said territory." This became known as the Wilmot Proviso.
The Wilmot Proviso was added as an amendment to an appropriations bill. It never became law; but succeeded in publicizing, and polarizing, the slavery issue permanently. It had last been an issue settled by the Missouri compromise; but had been waiting for the opportunity to rear its ugly head. The Wilmot proviso gave it the opportunity to do so. The Wilmot proviso came up for vote twice; each time the House passed it; but the Senate did not. In the meantime, John C. Calhoun (an old man at this point) offered a series of resolutions, the Calhoun Resolutions, to counter it. He argued that since the territories were the common possession of the states, Congress had no right to prevent any citizen from taking slaves into them, as this would be a violation of the 5th Amendment prohibition against the taking of property without due process of law. Calhoun’s resolutions never came to a vote; but he made his point, and his resolutions became the settled policy for the South and Southern congressmen.
The lines were now drawn. Senator Thomas Hart Benton said that Calhoun and Wilmot had fashioned a pair of shears. Neither one could do much by itself, but together, they could sever the ties of Union. Indeed they could. Wilmot’s position was cast in stone for the opponents of slavery; Calhoun’s for the South.
The Mexican-American War did not sow the seeds of the Civil War. Those seeds had already been sown with the spread of slavery in the South, with the fights that led to the Missouri Compromise, and with the Nullification Crisis. However, the Mexican-American War surely helped those seeds to grow and bear fruit.
The Mexican-American War did not cause the problems between the North and South. However, it exacerbated them. It did so because it forced the country to reopen the issue of slavery in a whole set of new territories. The country had (at least temporarily) resolved the issue with the Missouri Compromise. The taking of so much territory from Mexico reopened the controversy and allowed for more conflict.
So, the Mexican-American War did not sow the seeds, but it certainly helped them grow and did much to cause the Civil War.
I agree with post #7. How are we to discern a "seed" from "fertile soil" or "watering". It is all subjective to when one decides to begin their time-line. There is no closed environment and everything is connected. I like the domino metaphor. It is essentially true for any historical event.