One can argue that the Mexican-American was an unprovoked and unjustifiable war of aggression and territorial aggrandizement. It could also be argued that the war was not unprovoked nor was it territorial aggrandizement. Realistically the war provoked, justifiable, and not territorial aggrandizement. This notion is exemplified through the document provided.
A huge contributor to this war was the Lone Star Republic of Texas. Mexico refused to recognize the annexation of Texas. Mexico even stated that if the United States were to admit Texas to the Union it would be considered an act of war. The fact that Mexico refused to recognize the annexation of Texas, and threatened to go to war over it shows one of the justifications that the war was neither unjustified nor unprovoked.
The United States never tried to provoke the war actually quite the opposite. President Polk was negotiating for peace. Polk even stated in his letter to Senator William H. Haywood (August 1845) that all military and naval movement against Mexico be strictly defensive. Polk did not want to be the aggressor. He also made it clear that if the Mexican Army were to invade Texas (which was now part of U.S.) that the U.S. army would come to her aid, and if possible drive the Mexican army back to its territory. This is another fine example of how this war was provoked. Even though Mexico had declared war upon The U.S. we refused to take the offensive, and only acted in self-defense.
While at war with Mexico President Polk was still trying to make peace. President Polk with the consultation of his cabinet sent John Slidell to Mexico. Slidell was sent to with an important mission, and that was to “adjust a permanent boundary Mexico and the United States”. For this boundary adjustment to be made Slidell would have to purchase a portion of Upper California and New Mexico. This was thought to be a better boundary then from the Rio Grande’s mouth to the Passo. Polk was ready to pay forty million for this new boundary. Mexico refused this offer for peace. (Diary of James K. Polk September-October 1845)
... had never recognized its sovereignty. If we annexed a part of 'their' nation, they threatened a war. By not compromising, Polk ... over Texas. Texas considered itself an independent republic and wanted to be annexed by the United States, but Mexico ... refused to compromise with Mexico because we didn't consider them equals. Was Oregon any less a part of our Manifest Destiny than Texas ...
Because of Mexico’s refusal of boundary adjustments, the U.S. had no choice but to use force, make the boundary adjustments, and seize California. America’s hand was forced when Great Britain had their eye on California and intended to possess it. Due to the Monroe Doctrine President Polk and the United States could not willingly permit California to contain colonies of a foreign monarchy. With the terms of the Monroe Doctrine stating that any new colonies on the Western Hemisphere would be considered an act of aggression towards the United States. By using force to obtain California the Monroe Doctrine was upheld, and further war was prevented. (Diary of James K. Polk September-October 1845)
With the evidence exemplified throughout this paper The Mexican-American war was a war of just cause. It was not an unprovoked and unjustifiable war of aggression and territorial aggrandizement. It was war of Mexican aggression toward the U.S. fought in self-defense. Also fought to uphold the Monroe Doctrine and prevent further war with Britain.