The United States of America in the 1800’s was a rapidly growing country. Many people moved west for religious freedom while others were strong believers in Manifest Destiny. In order to occupy land, settlers had to find land that wasn’t already occupied. Since most of the west was a part of Mexico, this was not easy to do. Luckily, Mexicans also wanted settlers in their western lands because that area was not developed. Mexico invited Americans to settle in their land. However, both countries had many differences. It did not take long before the United States and Mexico went to war. The United States was justified in going to war because Mexico had shed American blood on American soil, Texas (a land that many Mexicans still considered theirs) was an independent republic and had the right to govern itself, and Texas was trying to become part of the United States, which means that the United States had a right to be involved, too.
Mexico had encouraged Americans to settle in the province of Texas, which was part of Mexico at the time. After the Texans and Mexicans fought, Texas won its independence from Mexico in 1836. Now, in 1846, America sent a troop of men lead by General Zachary Taylor to the Texan border to make sure the Mexicans weren’t attempting to attack Texas. This angered the Mexicans since it made it seem like Texas was now allied with America. The dispute that followed led to the death of sixteen Americans. Also, the Mexicans chose to believe that the arrival of General Zachary Taylor was an outright attack, which lead to the battles of Palo Alto and Resaca de la Palma.  This shows that Mexico had taken the first step of fighting the United States. Eventually, the Mexicans were forced to surrender. As to which side fired first, this cannot be determined. In one letter to Congress, James Polk stated, “We have tried every effort at reconciliation. The cup of forbearance had been exhausted even before the recent information from the frontier of the Del Norte. But now, …Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil”.
Before Texas became part of the United States, it was an independent republic. An independent republic has the right to govern itself and set itself apart from other nations. Texas regarded the "Rio del Norte" to be the Texan border. This means that Texas now has the choice to do whatever they wish. Charles Sumner, a state legislator from Massachusetts, said, “…in seeking extension of slavery, (our own citizens denied) the great truths of American freedom” . If Texas is independent, it has the right to own slaves. Also, states in the United States already owned slaves, so the point cannot be argued any further. Sumner also said Americans brought slaves in Mexico in defiance of Mexican law. However, Mexico invited Americans into Mexico and Mexico should have realized that Americans would have different ways. Also, Texas was trying to become part of the United States, which meant that the United States had a right to go to war with Mexico.
Since Mexico could not govern all of its territory, they invited Americans in. Now that Americans were settling in unpopulated Mexican territory, the ratio of Americans to Mexicans increased. Eventually, the American lifestyle took over, and demanded different rights. Velasco-Marquez said the treaty signed between Texas and the U.S. and also the fact that Texas wanted to be annexed to the U.S. was an act of war. Texas had the right to be a part of the U.S., and if Mexico considered this an act of war with Texas, then the U.S. had the right to be involved too . Finally, James Polk said, “The invasion was threatened solely because Texas had determined…to annex herself to our Union” . If that is true, then America did have the right to declare war.
Once Mexico showed aggression to Texas, the United States had a right to go to war with Mexico because Texas was an independent republic and wanted to be annexed into the United States. Whatever Texas does, Mexico cannot have a stake in it. From these points, it can be concluded that the United States was justified in going to war with Mexico.
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