The original "dark horse" candidate, Polk was initially considered only a vice-presidential contender but won the Democratic nomination for the 1844 election at the ninth ballot, on the basis of his expansionist policies. Like his mentor, Andrew Jackson, he endorsed the idea that his nation had a "manifest destiny" to rule from sea to sea. He supported the "re-occupation" of Oregon, the "re-annexation" of Texas, and the acquisition of California. These goals were largely achieved. The Oregon issue, and with it the question of the north-western border of the US, was peacefully settled with Great Britain. The Texas issue provoked war with Mexico, but American forces led by General Zachary Taylor won a series of resounding victories, even occupying Mexico City. By the end of Polk's presidency, the southern borders of Texas had been more or less settled in their present form, while Mexico had also ceded New Mexico and California in return for $15m.
These ostensible successes had their drawbacks. Polk's enemies denounced the Mexican war as an immoral war of aggression, and the vast expansion of US territory left a legacy of bitterness between North and South over the question of slavery in the new territories.
Polk, originally a North Carolina lawyer, retired at the end of his first term and died three months later, possibly from cholera, at his home in Nashville, Tennessee. A notorious workaholic, he was widely felt to have undermined his health by his obsessive labours in the White House.
In his own words
"No president who performs his duties faithfully and conscientiously can have any leisure."
"I am sure I shall be a happier man in retirement than I have been during the four years I have filled the highest office in the gift of my countrymen."
In others' words
"Politics had become his whole life, aside from which he had no aspirations, intellectual interests, recreation, or even friendships." Charles G Sellers
"I more than suspect that he is deeply conscious of being in the wrong – that he feels the blood of this [the Mexican] war, like the blood of Abel, is crying to heaven against him... He is a bewildered, confounded and miserably perplexed man." Abraham Lincoln
"James K Polk, a great president. Said what he intended to do, and did it." Harry S Truman
Polk was plagued with diarrhoea for much of his presidency.
When he was 17 he underwent an operation to remove his gallstones, without anaesthetic. (He drank alcohol to dull the pain.)
Aged 49 when he took office, Polk was the youngest president to date.
The Whig press denounced Polk as a coward for having once refused to take part in a duel.
The California gold rush began in his presidency.
His hairstyle – short at the top but long at the back – has been described as the first "mullet".
His wife, Sarah Childress, banned dancing in the White House.