The Remainder Louisiana Territory was all the land from a line drawn along 102° West of the original Louisiana Purchase that was still held by the United States of America after its defeat in the First American War. Settled in the Treaty of Liverpool in 1815, the British effectively cut off all American attempts to reach the Pacific for over 100 years. The line itself is usually refered as the Wellington Line after the Duke of Wellington that negotiated the treaty for Britain.
The land that was part of the Remainder Louisiana was merged with the Hudson's Bay Company territory of Rupert's Land, and for years it was namely a place that was to be used for exploitation, namely in furs and other resources that could be traded for. In 1869, when the nation of Assiniboia was created, Remainder Louisiana was merged with British Oregon, and immigration was finally allowed in the land. Small settlements sprung up as it was clear that the eastern Rocky Mountains was a treasure trove of resources including gold, silver, coal and iron. Towns like Denver sprung up overnight, and established an thriving, booming economy that attracted many new settlers and immigrants.
Many of the settlers in British Oregon where of American origin, which complicated matters as the US began to push for more rights, possibly even of succession or annexation of the areas with many American settlers. This lead to increasing tensions with Britain to the point that immigration was closed to Americans, and those settlers that were in Oregon had to become British citizens or be deported. After the Desert Massacre in 1881 that took place in the southern part of the Oregon Territory, this lead to a the Third American War, which led to an American defeat, and the continuation of British policies in Oregon.
It wouldn't be until 1916 after the Second Global War before the Wellington Line would be erased and America would claim the land to the west, and the "Great Migration" occurred as thousands flooded into Oregon with help from the US government, seeking to establish Oregon as part of America as quick as possible. By the Great Depression, which started in 1931, Americans outnumbered the former British subjects by two to one, and by the end of the Third Global War, the area was five to one American.
The territory would late become part of US States of Minnesota, Iowa, Missouri, Sequoyah, Nebraska and Dakota. Part of the remainder would become Mississippi, Louisiana, Arkansas, parts of Tennessee and Georgia would later become part of the Confederacy.