In 1818, the United States and the United Kingdom, which controlled British Canada, established a joint claim over the Oregon Territory, the region west of the Rocky Mountains and between 42 degrees north and 54 degrees 40 minutes north (the southern boundary of Russia's Alaska territory). The territory included what now is Oregon, Washington, and Idaho, as well as land up the western coast of Canada.

Joint control of the region worked for more than a decade and a half, but ultimately the parties set out to divide Oregon. Americans there outnumbered the Brits in the 1830s, and in the 1840s, thousands more Americans headed there over the famed Oregon Trail with their Conestoga wagons.

Belief in the United States' Manifest Destiny

A big issue of the day was Manifest Destiny or the belief that it was God's will that Americans would control the North American continent from coast to coast, from sea to shining sea. The Louisiana Purchase had just about doubled the size of the United States in 1803, and now the government was looking at Mexico-controlled Texas, the Oregon Territory, and California. Manifest Destiny received its name in a newspaper editorial in 1845, though the philosophy had been very much in motion throughout the 19th century.

The 1844 Democratic presidential candidate, James K. Polk, became a big promoter of Manifest Destiny as he ran on a platform of taking control over the entire Oregon Territory, as well as Texas and California. He used the famous campaign slogan "Fifty-Four Forty or Fight!"—named after the line of latitude serving as the territory's northern boundary. Polk's plan was to claim the entire region and go to war over it with the British. The United States had fought them twice before in relatively recent memory. Polk declared that the joint occupation with the British would end in one year.

In a surprise upset, Polk won the election with an electoral vote of 170 vs. 105 for Henry Clay. The popular vote was Polk, 1,337,243, to Clay's 1,299,068.

Americans Stream Into the Oregon Territory

By 1846, the Americans in the territory outnumbered the British by a ratio of 6-to1. Through negotiations with the British, the boundary between the United States and British Canada was established at 49 degrees north with the Treaty of Oregon in 1846. The exception to the 49th parallel boundary is that it turns south in the channel separating Vancouver Island from the mainland and then turns south and then west through the Juan de Fuca Strait. This maritime portion of the boundary wasn't officially demarcated until 1872.

The boundary established by the Oregon Treaty still exists today between the United States and Canada. Oregon became the nation's 33rd state in 1859.

Aftereffects

After the Mexican-American War, fought from 1846 to 1848, the United States won the territory that became Texas, Wyoming, Colorado, Arizona, New Mexico, Nevada, and Utah. Every new state fueled the debate about enslavement and which side any new territories should be on—and how the balance of power in Congress would be affected by each new state.