What does the phrase fifty-four forty or flight refer to?

The phrase "fifty-four forty or fight" was a slogan used during the 1844 U.S. presidential campaign, primarily by supporters of James K. Polk. It referred to the northern boundary of the Oregon Territory, which was claimed by both the United States and Britain, with "54°40′" being the latitude line that marked the U.S. claim. The slogan encapsulated the expansionist sentiment of the time, advocating for U.S. control over the entire Oregon Territory, though the boundary was eventually settled at the 49th parallel without conflict.