Tensions first heated up when the Mexicans, realizing that the Americans soon vastly outnumbered the 3000 Mexican ranchers in Texas, began imposing laws prohibiting or curbing immigration, abolishing slavery, and banning Protestant churches, three measures that the Mexican legislature hoped would discourage more Americans from immigrating, but in reality only served to increase tensions. This first in a long line of events would lead to the Mexican War, the first in a long line of events characterizing US contempt for sovereignty and intervention in Latin America.
These tensions reached a boiling point in 1834, when General Antonio Lopez de Santa Anna dissolved the Mexican Congress and declared Mexico a dictatorship. To enforce his new regime, Santa Anna sent an expedition north to Texas, where a small American militia captured and drove back Mexican troops in San Antonio. Under the leadership of Sam Houston, American Texans routed the Mexican army at the Battle of San Jacinto and captured Santa Anna, who was forced to sign an agreement recognizing Texan sovereignty and independence. Now a free republic, Houston nullified the Mexican anti-slavery laws and worked to incorporate Texas as a part of the United States as quickly as possible.
Unfortunately for Sam Houston, the US Senate was not necessarily in favor of annexing Texas. Politicians such as Henry Clay said that he would only support annexation if such a move could be done without any war with Mexico. However, everything changed when James K. Polk became president; decidedly pro-Texas, pro-expansionist, and pro-manifest destiny, Polk began making war preparations to annex Texas under any means necessary, including war with Mexico. To ensure that Great Britain would not interfere in the upcoming Mexican War, Polk made a treaty with Britain to divide the Oregon Territory between American and British land. This deal left Polk free to deal with Mexico.