While these three states and two territories were officially the extent of the United States, that was not all the land claimed by the country in 1840. The vast area lying to the north of Mexico and west of the continental divide, called by the Americans the Oregon Country, was in theory jointly administered by Great Britain and the United States, but by 1840, Americans were thinking that this was of necessity a part of their country.
The Treaty of 1818, between Great Britain and the United States, established the northern border of the United States for the lands gained by the Louisiana Purchase, with each country giving up a bit of land to the other. By the treaty, the border ran due south from the northwestern point of Lake of the Woods to the 49th parallel, and from thence straight west to the crest of the “Stony Mountains,” that is, the continental divide in the Rocky Mountains.
The treaty did not, however, continue the border west of the continental divide, for both sides felt that they had a strong claim to the lands between Mexico and Russian America. This area, called by the Americans the Oregon Country—-the British called it the Columbia District—-was the focus of a long simmering conflict between Great Britain and the United States.
For many years, the Pacific Northwest had been subject to differing claims by several countries—the British, Spanish, Russians and Americans. The Spanish claimed most of the land along the western coast of America based on their explorations in the 18th century. At the end of the century, Spain did grant Britain some rights in the area, but it wasn’t until the 1819 Adams-Onis Treaty, between the United States and Spain, that the latter agreed to a north border for its lands at the 42nd parallel. About the same time, Russia gave up its claims south of 54°40′.
This left the large area west of the continental divide and between 54°40′ and 42° to be disputed between Great Britain and the United States. Britain claimed the area because of its early exploration along the coast and because of overland explorations by British fur company men. The United States claimed the area because of Robert Gray’s discovery of the Columbia River in 1792 and the Lewis & Clark expedition, which reached Oregon Country in 1805.
Negotiations for the Treaty of 1818 did not resolve the dispute between the two countries for this area west of the continental divide.. The British saw this as a rich fur region and one that would limit the territorial expansion of the United States. The Americans saw Oregon as part of the natural lands that should be part of the United States by manifest destiny. Thus is was agreed that the region would have "joint occupancy" by the two countries.
This “solution” would not, of course, work in the long run. Initially, it was mostly British fur traders who were in the area, but in the 1830s, missionaries and settlers from the United States began to trickle into Oregon, the emigration reaching a steady stream in the 1840s. This lead to strong American support for annexing the entire Oregon Country, which was countered by British insistence of their control of all the lands north of the Columbia River. Neither country wanted to go to war (especially the United States which had just entered a war with Mexico, as will be discussed in the next blog in this series), so a compromise was reached in June 1846 to establish the border between the countries along the 49th parallel, extending this line across the continental divide from the east.
Thus it was in the fourth decade of the nineteenth century, the United States gained its northwest corner, encompassing today’s states of Oregon, Washington, Idaho and those parts of Montana and Wyoming west of the continental divide. This region was left as unorganized until 1848, when it established as the Oregon Territory, retaining this configuration until 1853. It was in this same decade, that the country also gained its southwest corner, as will be discussed in the next blog in this series.