President James Polk felt it his duty and legacy to achieve Manifest Destiny in his time as President. Besides the social and political rewards for expanding coast to coast, the economic rewards for taking Mexico's crown jewel, California, were very tempting for him.
While Mexico's army had inherited Spanish weapons and organization, and certainly gave Texas all it could handle during their war for independence, they had nod desire for war with the United States. Though they were bitter about Texas being annexed, they were willing to let it go to avoid armed conflict they had little chance of winning. What they weren't willing to stand for was the US claiming well beyond texas to the Rio Grande River. They offered a compromise to the Nueces River, but Polk wasn't looking for compromise, and bascially provoked them into a skirmish and a war.
So the factors were Manifest Destiny, Polk's political legacy, and the economic reward of California.
The real reason behind the start of this war was the desire of the US to expand its territory. This idea of "Manifest Destiny" drove the US's desire to have a war with Mexico.
There were also some more specific causes of the war.
- The US annexed Texas. Texas had become independent from Mexico about ten years earlier and Mexico did not recognize this. When Texas was annexed, Mexico felt that the US was taking part of its territory.
- The US tried to buy Texas and some other parts of Mexico and were rejected.
- Mexico sent troops into the disputed area between the Rio Grande and the Rio Nueces. This was seen by the US as an invasion of US territory.
The Mexican War (1846-1848) was fought between Mexico and USA. The causes of this war can be traced back to events that took place over previous two decades.
The Texas which was originally controlled by Mexico had been struggling to overthrow it. Finally in 1835 Texas revolted against Mexico and established an independent Republic of Texas in 1836.
Mexico refused to accept independence of Texas and warned that if Texas becomes part of USA it will declare war against USA. Though the tension between Texas and Mexico continued, it was not until 1845 that Texas became a part of USA. Mexico did not declare war against USA but broke off relations with it. Though there was no direct action, the tension between USA and Mexico remained.
As a result many different disputes that developed between the two nations which could not be resolved by peaceful means. One of the disputes was over demarcation of boundary between Texas and Mexico. Then USA wanted to recover from Mexico nearly three million dollar that it owed to US citizens as compensation for losses incurred by them in 1820's and beyond. Perhaps the most important cause of war was Desire of USA to expand southwards.
In 1945 USA sent an emissary to with a proposal to settle their claims and disputes. These proposals included purchase of New Mexico and California by USA. However due unstable political conditions within Mexico no talks could be held. The USA by this time had perhaps made up its mind to settle the dispute with Mexico by War. In 1946 USA orders a small army of 3000 men to advance into the disputed territory between Mexico and Texas. A small body of cavalry of this army was defeated by a much larger force of Mexico. This gave USA the opportunity to declare war.
When the United States annexed Texas, Mexico broke off diplomatic relations. Polk ordered General Zachary Taylor to defend the disputed border region between the Nueces River and the Rio Grande. Polk also sent John Slidell on a secret mission to Mexico to try to obtain the disputed area by negotiation. The Mexicans rejected Polk's offer to buy the territory in question as well as part of New Mexico and California. Mexico also reasserted its claim to all of Texas. A Mexican attack on American troops north of the Rio Grande provided Polk with the pretext to declare war. Although smaller, the American force was better led and supplied. The outcome of the war was never in doubt.