In 1845 the U.S attempted to Annex Texas. Basically America wanted Texas to become one of the states rather then and independent nation by itself. At this time Texas was an independent nation that was not a part of America or Mexico. Mexico wanted to keep Texas neutral if not a part of its own country. When the U.S attempted to annex Texas Mexico became outraged, " In November 1843 Mexico had warned that if the United States should commit the 'unheard-of aggression' of seizing an integral part of 'Mexican territory' Mexico would declare war " (Bound for the Rio Grande, 62). Despite the warning the U.S attempted to annex Texas. In doing so Mexico retaliated by breaking off all diplomatic relations with the U.S. Mexico felt that the U.S was insulting them by not taking them seriously when they threatened with war. So at this point America showed a very large interest in possessing Texas. America was very close to actually acquiring Texas when they made their first mistake in the war.
The U.S requested the also be given the Rio Grande. The Rio Grande was south of Texas and clearly belonged to Mexico (refer to figure 1). The U.S felt that the Rio Grande was part of Texas and should be given to the U.S with Texas. Mexico would not give up the Rio Grande because they were certain that the Rio Grande belonged to them. America's greed is THE major reason this conflict occurred. The U.S also believed the Mexico should have to pay for any of the U.S's debts that were incurred during Mexico's conflicts with Spain. America was in debt 3 million dollars because of the Mexico and Spanish conflicts and America very strongly believed that these debts were Mexico's fault and they should have to pay for them. Mexico felt like it was Americas fault that they had to pay anything and they [Mexico] should not have to pay " Mexico felt no obligation to pay and they felt as though America was attempting to use them as a source of income" (The Texas Military Experience, Website)...