All of Mexico
After the election of Polk, but before he took office, Congress approved the annexation of Texas. Polk moved to occupy a portion of Texas which had declared independence from Mexico in 1836, but was still claimed by Mexico. This paved the way for the outbreak of the Mexican-American War on April 24, 1846. With American successes on the battlefield, by the summer of 1847 there were calls for the annexation of "All Mexico", particularly among Eastern Democrats, who argued that bringing Mexico into the Union was the best way to ensure future peace in the region.