James K. Polk was born in Mecklenburg County in 1795, North Carolina. He graduated with honors in 1818 from the University of North Carolina. He continued to become lawyer and eventually got involved in politics by joining the TN legislature. He then became the Speaker of the House in the House of Representatives from 1825 to 1839 and then the TN governor.

James K. Polk has been referred to as the "Dark Horse" and was the last of the Jacksonians in office and is arguably the last strong president in office before the Civil War. He was also a good friend of President Andrew Jackson. He was also the "Chief Lieutenant" in Jackson's Bank War. 
Leading into the Election of 1844, Polk was the lead contender for Vice President under Martin Van Buren for the Democratic party. Polk took the stance that we should annex Texas as well as Oregon while other candidates were opposed.
 

James won by a small margin of popular votes, but dominated the votes from the Electoral College. Even before he was put into office, Congress passed a "joint resolution" that offered annexation to Texas which Polk was in favor of. This created potential conflict with Mexico over Texas, who had claim over the land at the time.

A risk of war arose when Polk wanted to take claim Oregon which belonged to Great Britain. Thus, "54-40 or fight!" was born (boundary of California 54-40 latitude and northward). Thankfully, they were mutually against starting war. The British agreed to settle for the 49th parallel, and the Treaty of 1846 was written and signed.

To obtain California and New Mexico Territory from Mexico, Polk offered up to $20 million, but this offer was denied. Polk declared war - Mexican American War - in 1846. Ultimately, Mexico cede the land in 1848 to the US.

James Polk only served one term and decided not to run for a second. He believed that he accomplished what he wanted to as far as expansionism. Acquiring present day Oregon, California, New Mexico, Texas, Arizona, Colorado, and Nevada was no easy feat in the four years that her had during his term. But through war and negotiation he made it possible. Three months after he was out of office, he died from Cholera.