The Oregon Country had been claimed by various nations, including Spain, Russia, Great Britain and the U.S., but the U.S. and Great Britain soon made a joint occupation agreement, in 1818, until it was superseded by the Oregon Treaty of 1846, which established it as a U.S. state at the 49th parallel. Numerous free-state politicians vied for acquiring Oregon Country, in order to balance out the annexation of Texas, a slave state at the time.
While Oregon was initially a jointly settled land between the British and Americans, the two citizen groups were very different. British settlers worked in the fur trade, while the Americans came from a diverse range of groups.
When news of Oregon's fertility and opportunity became popular, many Americans began to travel to Oregon to settle there. This led to a popularization of the Oregon Trail and a huge population disparity between the British and quickly-settling Americans. The fur trade also began to decline, and since the British wanted to avoid war with America over the issue, a treaty was soon signed granting full United States ownership of the state of Oregon and officially establishing the state's borders.
The acquirement of Oregon was also part of the newly elected president James K. Polk's policy towards aggressively expanding the United States. Besides acquiring Oregon and Texas, Polk also bought California from Mexico. The desire of politicians to annex more states prompted escalations that lead to the Mexican-American War and subsequent further acquisition of additional states.