When Texas broke away from Mexico in 1836, the majority of Mexicans believed it was still a part of Mexico. The government of the United States believed differently. As his time in office was coming to an end, President John Tyler received enough support to annex Texas. This was in no small part due to the Manifest Destiny. At this time, Americans were constantly looking for land to start farms or sell to incoming farmers. They found the most easily accessible and available land in Texas. There was also the ideal that the Southerners traveled into Texas in order to get another slave state into the Union.
When the annexation occurred, Mexico broke relations and withdrew its minster from Washington. President James K. Polk believed that he had to restore diplomatic relations, so he sent a special temporary envoy to Mexico. The Mexicans expected that envoy John Slidell would make an offer for Texas, after which Mexico would receive him or someone else as permanent minister. Instead Polk made Slidell permanent minister and instructed him to open negotiations for the sale of California. Mexicans thought that if they made Slidell a permanent minister, they would never be paid for Texas. Polk had backed up Slidell by sending troops to the Rio Grande, which Texans claimed was their proper boundary. The Mexican president, José Herrera, was newly in office and feared being overthrown if he complied with Polk. Slidell left Mexico City for a town a few miles away and Herrera sent troops to the Rio Grande to confront the Americans. Polk and his cabinet prepared a declaration of war. He only expected to fight a few battles on the Rio Grande and then start negotiating. Polk was very anxious to purchase California from Mexico and he knew that Mexico would refuse to give it up. He feared that England would seize it as a strong point on the Pacific. .
The United States dominated Mexico throughout the entire war.
