What were James K. Polk's successes and failures as president? What impact did his administration have on the future of the United States?

James K. Polk took office as the eleventh President of the United States and served from 1845 to 1849. Before becoming President, Polk served as Speaker of the House of Representatives and later became Governor of Tennessee, both experiences which influenced his performance in the White House. Polk ran for President after being considered as the Democratic nominee for Vice President in 1844. He chose to pursue the presidency to oppose Henry Clay, who was opposed to the re-annexing of Texas and Oregon.
Polk's presidency was characterized by his commitment to “Manifest Destiny,” a concept which held that America should seek to expand as much as possible. He was nominated on the ninth ballot and received support from such prominent figures as President Jackson. Despite the popularity of his campaign to expand United States territory into Texas, Oregon, and California, Polk's presidency was not without controversy. In his attempt to acquire Oregon, Polk put the country at risk of another war with Great Britain, which was considered a failure by many of his contemporaries.
One of Polk's greatest failures was the decision to send an envoy to offer the Mexican government a maximum of $20 million in addition to settlement claims in return for New Mexico and California land. This attempt was a resounding failure and the envoy Polk sent was not received. Polk's further attempts to acquire a response yielded an attack from Mexican forces and led to all-out war.
Although the Mexican envoy was one of Polk's greatest failures, it was also one of his greatest successes in the sense that it resulted in the acquisition of New Mexico and California for $50 million less than the country had been prepared to offer. Polk is also notable for his success as a “dark horse” candidate, whom no one expected to win the presidency in 1844.
Despite being a primary force in the acquisition of modern-day Oregon, California and much of Texas, Polk is one of the lesser known presidents. Nonetheless, his actions as President of the United States played a significant role in the tensions between the North and South that led to the Civil War. The territories he acquired are by far the most lasting impact his administration had on the future of the United States, and his decisions also served as a catalyst for the Civil War.
 
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https://www.whitehouse.gov/about-the-white-house/presidents/james-k-polk/