Upon first examination, it appears that President Polk, with his aggressive promises and desire to gain California at all costs, caused the war. However, if one looks back one term to the previous President, one can see the foretelling of the Mexican-American War. Realizing his unpopularity with both major political parties, President Tyler saw the annexation of Texas as a ticket to a second term. If he could make the American people identify the name Tyler with American expansion, maybe he could overcome the disdain of both the Democrats and the Whigs (Combs 88). Unfortunately for Tyler, Polk won the election. Wanting to finish what he started, and perhaps to make a name for himself among America's Presidents, Tyler made use of joint resolution of Congress for the annexation rather than a treaty, which would have required a two-thirds majority that Tyler did not have (Combs 89). Finally, on the last day of his term, Tyler sent messengers to Texas for the purpose of immediate annexation. Mexico was not pleased.
Now, the outcome of the above events could have been easily avoided by the new President. If he would recall the messengers and agree to negotiate a new treaty, Mexico would be placated (Combs 89). Polk did no such thing and Mexico declared that the annexation was an act of war. Premonitions of the Mexican-American War can actually be found in the lame duck days of President Tyler's administration.
The actions of the President of the United States have a profound impact on America's foreign relations. However, in some instances, the actions of the people have an even more profound impact. This was certainly the case with the Mexican-War. The settling of Texas and other western areas played a major role in the war that would be fought between America and Mexico. But why would these settlers knowingly move west of America's present boundaries, and even into foreign territory? The answer is simple: economics. By moving ahead of "official" settlement in the U.S. proper, these Americans were hoping to "get in on the beginning of the price rise" (Lavender 127). In other words, these people wanted to sell the land at high prices when it became part of the United States. There were also plentiful trade opportunities as well as a distressing power vacuum out west. What made these settlers and traders think the land would one day be part of the America? History up to that time showed how America had constantly expanded westward across the continent. America showed no signs of stopping anytime soon. The idealistic concept of Manifest Destiny also convinced people to move West. For them, certain parts of North America were simply "destined" to become part of the United States; the French, English, Spanish, Mexicans, Russians and the Indians be damned (Newhouse 142).
The forward thinking settlers simply presumed that America would eventually assume their new lands. Then they could sell the land at higher prices to farmers, plantation owners, or whomever when the land finally was part of the United States. In effect, these settlers were waiting for America to "catch up" to them (Lavender 127).
What is role of President Polk and Tyler in all of this? Surely, their actions played a large role in the Mexican-American War. This fact cannot be denied. But what was the motivation for these actions? Why did Tyler want Texas so badly? Obviously, he wanted to be the President for four more years. But it goes deeper than that. The question of Texan annexation would not have come up if there had not already been Americans settling and causing problems (from the Mexican perspective) in Texas. These settlers were in Texas due to the reasons mentioned earlier: economics and the arrogant Manifest Destiny. Why did Polk promise California and Oregon to the voters? He did this because Americans were already there and because of the massive economic potential in these areas.
The westward movement of Americans and Manifest Destiny have been established as the overall causes for the Mexican-War. However, it is important to examine, and perhaps refute, some other possible causes of the war.
During and after the war, many in the United States placed the majority of the blame for the Mexican-American War squarely on the shoulders of Mexico. There may be a grain of truth in this ultra-patriotic view (Combs 99). President Polk sent troops under General Zachary Taylor to the region between the Rio Grande and Nueces Rivers. Texas believed that its southern boundary was represented by the Rio Grande River. The Mexicans, however, did not acknowledge this boundary and instead believed that it was the Nueces River. So, the Americans believed they were on Texan (soon to be American) soil, while the Mexicans believed that the Americans were on Mexican soil (Lavender 130). When Mexican forces attacked the Americans in this region, Polk believed that Mexico "invaded our territory, and shed American blood upon the American soil" (Richardson 442). With this information in hand, Polk proceeded to ask the Congress for a declaration of war, which he received easily. However, according to Polk's diary and other sources, he planned to ask Congress for a declaration before word of the Mexican "attack" ever reached Washington (Quaife 386). Refuting this "Mexico's Fault" theory even more is the fact that the government of Mexico at this time was in a period of chaos (Garraty and Gay 811). Still, the attack proved an effective scapegoat for not only Polk, but many other pro-war politicians.
The Mexicans can not be totally blamed for this war. The attack that many patriots were so fond of using as justification for the war was simply an attempt by Mexico to defend land that it believed belonged to Mexico. Of course, Mexico could have responded in a more peaceful manner to the idea of Texas annexation. And Mexico could have responded better to American desires to purchase California and other areas of the Southwest (Lavender 130). If only it could be as easy as the Louisiana Purchase had been, Polk must have thought.
Another suspected cause of the war is the desire of the southern states to gain more slave states, thereby increasing their political power. Those in the free states to the north tended to hold this view of the war. Many of the Americans that had moved into Texas ignored the slavery restriction. Northerners feared that Texas would join the Union as a slave state, since there were obviously already slaves in Texas. It was this very fear that Calhoun had used to ensure that President Tyler's annexation treaty with Texas would not get the two-thirds majority needed in the Senate for approval (Combs 88). But if the southern states wanted Texas so badly, they had a strange way of expressing it. When the call went out for volunteers to join the military in the fight against Mexico, most came from the western states and even the territories (Lavender 130). If the south was so interested in gaining Texas as a slave state, one would think that they would have sent more troops than they did (Newhouse 142).
Many probable causes of the Mexican-American War have been posed throughout our nation's history. These range from the obvious (Mexico) to the subtle (southern "slave power"). The two causes that make the most sense, however, are the constant westward movement of Americans and the concept of Manifest Destiny.
(Mario's note: I found this article on the web and found it to be quite accurate. It was written by an American High School student.)