The slogan "54-40 or Fight" referred to the latitude line of 54 degrees 40 minutes, which was the northern boundary of the Oregon Territory. The United States and Britain had jointly occupied the territory for decades. Both nations felt they had a legitimate claim to the land and its wealth of resources.
This excerpt is from President James Polk's 1845 inaugural address.
Nor will it become in a less degree my duty to... maintain by all constitutional means the right of the United States to that portion of our territory which lles beyond the Rocky Mountains. Our title to... Oregon is "clear and unquestionable," and already are our people preparing to perfect that title by occupying it with their wives and ascended the Missouri to its headsprings, and are already engaged in establishing the children.. [0]ur people... have filled the eastern valley of the Mississippi, To us belongs the duty of protecting them adequately wherever they may be upon our blessings of self-government in valleys of which the rivers flow to the Pacific.. soil. The jurisdiction of our laws and the benefits of our republican institutions should be extended over them in the distant regions which they have selected for their homes.