From The Company: The Rise and Fall of the Hudson's Bay Empire, by Stephen Bown (Doubleday Canada, 2020), Kindle pp. 280-281:
The European colonial settlement of eastern North America had progressed quickly in the last decade of the eighteenth century and into the nineteenth century. Cities like Boston, New York and Philadelphia had mushroomed after the revolution, and farmland expanded to feed the influx of people and increasingly encroached on the traditional territories of Indigenous peoples. The British, anxious to maintain against the United States a legal claim to the Columbia River, the artery of the fur trade west of the Great Divide, proposed extending the 49th parallel west to the Columbia and then following the Columbia as the border to the sea. To the American negotiators who had their eye on the large, deep harbours of Puget Sound (the only viable harbours for large ships north of San Francisco) this was not ideal. But in 1818, weary from years of inconclusive conflict during the War of 1812, neither the British nor the Americans were willing to grapple over who would lay claim to the land on the far side of the Continental Divide. So they agreed to jointly “occupy” the region, deferring more complicated, and politically charged, questions to the future. (The terms of the Convention of 1818 were reaffirmed indefinitely in 1827, with the provision that either country could cancel the agreement with one year’s notice.)
In February 1819, the United States and Spain signed the Adams–Onis Treaty. In addition to selling the territory of Florida for $5 million, Spain also agreed to the northern boundary of California being set at the 42nd parallel and ceded any rights to the territory north of that to the United States. Russia, in two separate treaties—with the United States in 1824 and with Britain in 1825—bowed out of Old Oregon (but retained the right to trade in the region), agreeing to a southern boundary for Alaska roughly similar to the Canadian-American border today.
Old Oregon, now defined as the territory west of the Rocky Mountains, north of Spanish California and south of Russian Alaska, became a political no man’s land, jointly claimed on paper by Britain and the United States, and open to settlement and commercial development from either nation, although neither had any tangible presence there and they had neglected to inform the local inhabitants of their decision. Of course, the only commercial development was the fur trade, and the traders were more likely to follow the customs of their Indigenous hosts and customers than those of Londoners, Montrealers or New Yorkers. The vast territory remained unchanged for decades, until the 1830s, when the first wagon trains began rolling west along the Oregon Trail.
The Hudson’s Bay Company and the North West Company faced other challenges east of the Rockies that proved to be more of a threat—their own internecine quarrels.