Was the Expansion of Slavery the Root Cause of the Mexican-American War and the Decision to Annex Texas?
During the annexation of Texas and the Mexican-American War, the so-called "conscience" Whigs, opposed to the United States' involved in these actions, attacked them as attempts to expand slavery. After all, the land gained with the annexation of Texas and gained from Mexico due to the war would almost certainly be open to slavery. However, other reasons, not the least of which were the doctrine of Manifest Destiny and the desire to gain an entrance into the Pacific, were more important factors in Presidents John Tyler's and James K. Polk's decisions to annex Texas and go to war with Mexico.
During the times of the Tyler and Polk Administrations, slavery was creeping up as a central issue in American national politics. The Northern free states were intent on keeping slavery from expanding further while the slave-owning South was focused on protecting its "right" to own slaves through power exercised in the Congress. However, since Henry Clay's famed Missouri Compromise of 1820, a precarious balance existed between North and South; each consisted of the same number of states and therefore controlled the same number of Senate seats. The annexation of Texas and the land gained from Mexico (the Mexican Cession) in the war threatened to tip the scales in favor of the slave states. Even more damning was the fact that both presidents were Southern Democrats at heart, with much to gain by pleasing their Southern power-base.
Of much greater importance to the United States government in gaining these territories, however, was the spreading of American democratic ideals. According to the precepts of Manifest Destiny, the United States had a "God-given duty" to enlighten the "savages" it shared the continent with by bringing democracy to them. Mexico, populated by Hispanic Catholics, was therefore seen as inferior by the WASPish American populace.
The use of Manifest Destiny as a justification for expansions is further proven by President Polk's and Secretary of State Daniel Webster's handling of border disputes with Great Britain over Maine and Oregon. Instead of going to war against Britain as the United States did with Mexico over border disputes with Texas and sparking a violent conflict, the United States settled both of these controversies peaceably. Polk and Webster both compromised in order to preserve the peace: Polk ceded to Britain some of Oregon Country in the Oregon Treaty of 1946 and Webster signed the dispute-settling Webster Ashburton Treaty, allowing the United Kingdom control over half of a disputed section of Maine. In both of these instances, America did not go to war with a nation that held similar "enlightened" ideals, institutions and a WASPish population over border disputes as serious as the one with the "inferior" Mexico.
On the other hand, it can be argued that the United States went to war with Mexico, rather than Britain, for other reasons. Mexico was a much weaker opponent than the current world power. Despite this seemingly sound logic, America had defeated Britain in the Revolutionary War and fought it to a stalemate in the War of 1812. The supremely arrogant James K. Polk would not hesitate over standing toe-to-toe with a foe that the United States had, in his view at least, defeated in the past.
The reasons that the two presidents gave for their expansionist policies also show that slavery was not foremost on their minds. Tyler simply grabbed at an opportunity by annexing the independent Republic of Texas. He wanted to prevent Europe from gaining a dominating influence on the fledgling nation and using its land to produce cotton to offset the tariffs imposed on its cotton. Furthermore, Polk only went to war with Mexico in order to gain California. He saw California as America's vital entry into the Pacific and would use any means necessary, even war, to secure such a vital treasure. The Mexican-American War was fought to gain important ports, not future slave states.
Texas was not annexed nor was the Mexican-American War fought solely to spread the "peculiar institution" of slavery. Though both Presidents Polk and Tyler had much to gain in pleasing their Southern, slave-owning power-base by admitting more slave states into the Union, slavery was not foremost on their minds. Of far greater importance to the two men were the spreading of the democratic institutions of America, preventing Europe from gaining influence in the Americas in defiance of the Monroe Doctrine, and gaining an access into the Pacific Ocean.
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