Britain and the United States agreed to what has since been described as “joint occupancy” of lands west of the Continental Divide to the Pacific Ocean as part of the Anglo-American Convention of 1818, which established the 49th Parallel as the international boundary west from Lake of the Woods to the Rocky Mountains. Resolution of the territorial and treaty issues, west to the Pacific, were deferred until a later time. Spain, in 1819, ceded their rights north of the 42nd Parallel to the United States, although these rights did not include possession.
Negotiations with Great Britain over the next few decades failed to settle upon a compromise boundary and the Oregon boundary dispute was highly contested between Britain and the United States. Disputed joint-occupancy by Britain and the U.S. lasted for several decades. With American settlers pouring into Oregon Country, Hudson’s Bay Company, which had previously discouraged settlement because it conflicted with the fur trade, reversed its position in an attempt to maintain British control of the Columbia District.
Fur trapper James Sinclair, on orders from Sir George Simpson, Governor of the Hudson’s Bay Company, led some 200 settlers from the Red River Colony west in 1841 to settle on Hudson Bay Company farms near Fort Vancouver. The party crossed the Rockies into the Columbia Valley, near present-day Radium Hot Springs, British Columbia, then traveled south-west down the Kootenai River and Columbia River. Despite such efforts, Britain eventually ceded all claims to land south of the 49th parallel to the United States in the Oregon Treaty on June 15, 1846.
In 1836, a group of missionaries, including Marcus Whitman, established several missions and Whitman’s own settlement Waiilatpu, in what is now southeastern Washington state, near present day Walla Walla County, in territory of both the Cayuse and the Nez Perce Indian tribes.
Whitman’s settlement would in 1843 help the Oregon Trail, the overland emigration route to the west, get established for thousands of emigrants in following decades. Marcus provided medical care for the Native Americans, but when Indian patients – lacking immunity to new, “European” diseases – died in striking numbers, while at the same time many white patients recovered, they held “medicine man” Marcus Whitman personally responsible, and murdered Whitman and twelve other white settlers in the Whitman massacre in 1847. This event triggered the Cayuse War between settlers and Indians.
Statehood:
The growing populace of Oregon Territory north of the Columbia River formally requested a new territory, which was granted by the U.S. government in 1853. The boundary of Washington Territory initially extended farther east than the present state’s, including what is now the Idaho Panhandle and parts of western Montana, and picked up more land to the southeast that was left behind when Oregon was admitted as a state. The creation of Idaho Territory in 1863 established the final eastern border.
Washington became the 42nd state in the United States on November 11, 1889.