From the research detected in the articles and in the book, both the U.S. and Mexico are at fault for the start of the Mexican War.
In 1820, some Americans led by Stephen Austin went to Texas to settle. The Mexican Government said they could settle there as long as they obeyed two laws, one was to pay the Mexican taxes and the other was to convert their religion to Catholicism. By 1833, there were about 35,000 American settlers in Texas. They weren't obeying the laws either. The Mexican government was against the Texas settlers bringing slaves into Texas, but the Americans did anyways because they didn't want to waste $1,000. Santa Anna the new dictator of Mexico began to clamp down on the Texas settlers and threatened to use Military force to make the Americans obey the laws. In 1836, Santa Anna led several thousand troops to meet the 150 Americans and the Americans had to take refuge inside the walls of the Alamo. Santa Annafirst bombed the inside, and then sent troops climbing over the walls to kill the Americans.
Santa Anna was now in pursuit of Sam Houston and his 700 Texans. The forces met at San Jacinto. The Americans pulled off a surprise attack and won. Santa Anna was captured and signed the treaty that gave Texas its independence. On March 1, 1845 Texas tried to become part of the U.S. John Tyler tried to convince Congress and finally Texas was annexed as a slave state. Mexico had a problem with the boundaries though. Mexico still thought they had control of Texas, even after losing at San Jacinto. Mexico then stopped talking with the U.S. The U.S. regarded war with indifference, while Mexico had inability to inflict serious injury upon it. The U.S. sent Sidell to Mexico City to talk about the border, but Sidell was not received. Mexico wanted Nueces River as the boundary, but the U.S. wanted the Rio Grande as the boundary. Polk ordered troops to North Bank of the Rio Grande. 63 Americans w