Polk was born in Mecklenburg County, NC. As a democrat, Polk served as the 17th Speaker of the House and Governor of Tennessee. He became the "dark horse" candidate and defeated Henry Clay to become the 11th President of the United States. Polk led the nation to a sweeping victory in the Mexican-American War, which gave the US most of its present SouthWest States.
James K. Polk was a clear underdog in the 1845 election, he ran against Henry Clay. His nickname was the "dark horse" because nobody expected him to win the Presidency. He only served for one one term with his VP being George Dallas and was succeeded by Zachary Taylor.
Polk's Presidential campaign ran on the idea of annexing Texas, which is the main reason why he was voted into office. He was the leader of our country during the Mexican-American War and bought California and New Mexico from Mexico for $15 million. So his presidential fundamentals revolved around expanding the US and annexing Texas.
In addition though, he fulfilled the "Manifest Destiney" dream by expanding America westward. Also he was known for not running for a second election because he believed that he accomplished everything that he wanted to in his first term.
Polk retired after his first term of office and died at the age of 53, passing away 4 months after he finished his term of Presidency. To this date, he is the President to have the shortest "retirement term" after serving as president. He was claimed to have died from stress and exhaustion of running the United States of America.
James K. Polk is actually considered to be one of the greatest unknown Presidents of the US. Many people saw him as a man who seemed bloodshed but he was quoted to "always stay true to his word" (periodic-presidents.com). He also proposed many bills to free slavery in also of the new founded western states, but they later died in congress. In addition, after Polk died he left in his will that all of his slaves were to be set free (millercenter.org)
In my opinion though, James K. Polk was an essential President because he expanded the United States by a near million square miles. In addition he promised if he was elected President I'm his first term and accomplished everything that he wished to do then he wouldn't run for another term, and he stayed true to his word.