British and American fur traders in the Oregon country—an indefinite term for the Pacific Coast between Spanish-held California and Russian-held Alaska—could send their pelts only by ship because of the lack of an overland route. In 1805 an American expedition under Meriwether Lewis and William Clark reached the mouth of the Columbia, having discovered an overland route from the East to the Pacific. They spent the winter in quarters called Fort Clatsop, on the south bank of the river near present-day Astoria.

David Thompson, a Canadian fur trader, explored the Columbia River for the North West Company during 1807–11, and planned to establish a post at the mouth. Before he could act, however, John Jacob Astor of the American Fur Company built a trading post, Astoria, at the site. After the outbreak of the War of 1812, the British seized Astoria.

In 1818 the United States and Britain signed a treaty agreeing to joint occupation of the Oregon country. The Spanish claim to the area was relinquished in 1819 by a treaty with the United States that drew the boundary between Oregon and Spanish California at the 42nd parallel. The Russian claim was resolved in treaties with the United States in 1824 and Britain in 1825, the boundary of Russian Alaska being established at 54°40' north latitude.

Meanwhile, in 1821, the Hudson's Bay Company absorbed the North West Company and took over the Oregon fur trade. For more than 20 years the only government in the region was that provided by the company factor (agent), John McLoughlin, often called the “Father of Oregon." A new post, Fort Vancouver, was built on the north bank of the Columbia in 1825. It served as company headquarters in the region and was the chief settlement in the Pacific Northwest.