Throughout the 1820s, Americans settled in the vast territory of Texas, often with land grants from the Mexican government. However, their numbers soon alarmed the authorities, who prohibited further immigration in 1830. In 1834 General Antonio López de Santa Anna established a dictatorship in Mexico, and the following year Texans revolted. Santa Anna defeated the American rebels at the celebrated siege of the Alamo in early 1836, but Texans under Sam Houston destroyed the Mexican Army and captured Santa Anna a month later at the Battle of San Jacinto, ensuring Texan independence.
For almost a decade, Texas remained an independent republic, largely because its annexation as a huge new slave state would disrupt the increasingly precarious balance of political power in the United States. In 1845, President James K. Polk, narrowly elected on a platform of westward expansion, brought the Republic of Texas into the Union. Polk’s move was the first gambit in a larger design. Texas claimed that its border with Mexico was the Rio Grande; Mexico argued that the border stood far to the north along the Nueces River. Meanwhile, settlers were flooding into the territories of New Mexico and California. Many Americans claimed that the United States had a "manifest destiny" to expand westward to the Pacific Ocean.
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