The phrase "54 40 or Fight!" or "Fifty-four Forty or Fight!" was the famous 1844 presidential campaign slogan of James Knox Polk that contributed to his unexpected victory. The slogan was named after a line of latitude that served as the northern border of Oregon at 54 degrees 40 minutes.
The Treaty of 1818 was established for the United States and the United Kingdom to have joint control over Oregon. The treaty lasted for a decade and a half and was dissolved when both parties decided to end the joint occupancy and divide Oregon.
James Knox Polk, considered the first "dark horse" president, correctly intuited that the American public favored expansion and wanted the entire territory of Oregon for the U.S. alone. He ran for the U.S. presidency under the Democratic Party using the slogan "Fifty-four Forty or Fight!" to convey his intention to either claim Oregon for the U.S. or go to war with the British.
Polk won by popular vote against Whig Party candidate Henry Clay. He was resolute about serving only one term and quickly set about fulfilling his campaign promises. To settle the dispute over the occupancy of Oregon, he offered to extend the Canadian territory along the 49th parallel, which the British initially declined, but later accepted after some minor modifications.