Polk did not really want to fight Britain. Instead, in 1846, he agreed to a compromise. Oregon was divided at latitude 49°N. Britain got the lands north of the line, and the United States got the lands south of the line.
War was also threatening with Mexico, so he agreed to a reasonable compromise with Britain over Oregon and pushed for war with Mexico. He also probably preferred a war with Mexico to a war with Britain.
Much of Oregon was already a territory of the United States. The problem was the location of the northern border between Oregon and the British territory of Canada and its exact location was a campaign issue in 1844.
threatened to go to war
The Oregon Territory was split between the United States and Britain.
Britain
The United States, Great Britain, Spain, and Russia claimed the Oregon territory, in other words, the Oregon Country.
In 1846 under President James K. Polk the U.S. signed a treaty with Britain that gave the U.S. possession of the Oregon territory.
Britain
No.
Great Britain
Britain
Oregon
Oregon Countrythe Oregon Countrythe Oregon Country
The Oregon territory