James K. Polk from Tennessee was the 11th President of the United States. He was known as the "dark horse" candidate because he was not expected to win. In fact, he actually intended to be vice president but became a presidential nominee partly because he supported adding Texas as a state.
President Polk was a strong believer in westward expansion. During his tenure as President, the United States gained western territories that are now part of these states: Arizona, California, Oregon, Washington, Idaho, Nevada, Utah, Texas, much of New Mexico, and parts of Wyoming, Montana, and Colorado.
Texas leaders wanted the new independent Texas Republic to become a U.S. state. But, the United States was not willing to admit Texas. One reason was because Mexico said it would declare war against the United States of Texas was grant statehood, and United States wanted to avoid that. The other key reason was Northerners in the United States did not want to admit Texas to the union because it would be a slave state, and most northern states opposed slavery.
For nine years, Texas was its own independent country. US government officials tried to annex Texas, but Congress refused. Tired of waiting, Texas leaders said that if the United States would not welcome Texas into the union, maybe France or England would!
President James K. Polk, however, promised to expand the country. With his support, Congress finally acted. Texas was annexed by the United States and became a state on December 29, 1845. Texas was the only state to enter the US after being a nation all by itself.
Both American and British fur traders hunted and traded in the Oregon Territory in the Northwest. At the Convention of 1818, Great Britain and the United States agreed to share control of the land. The area was shared for more than 25 years.
President Polk soon wanted West Coast land solely for the United States. He wanted to claim the Oregon Territory all the way to its northern boundary. Polk threatened war against Great Britain over Oregon.
Perhaps United States would’ve gone to war with Britain for a third time, but in 1846, the United States entered the Mexican War. To avoid a second war, the United States compromised with Great Britain. The two countries signed the Oregon Treaty, splitting the Oregon Territory at the 49th parallel. Great Britain received the northern part, and United States received the southern part.
President Polk is considered one of the most successful presidents of the first half of the 19th century, having reached nearly all his goals. Westward expansion was a clear success as the country going to huge amounts of territory. He is also credited with the strengthening the role of the president. What westward expansion produced a troublesome side effect: divisions over slavery‘s possible expansion set the stage for future conflict. Some people claim the president Polk was not fully sensitive to these deep divisions.