The southern boundary of the United States with Mexico was not the only western territory under dispute. The OREGON TERRITORY spanned the modern states of Oregon, Idaho, and Washington, as well as the western coast of Canada up to the border of RUSSIAN ALASKA. Both Great Britain and America claimed the territory. The TREATY OF 1818 called for joint occupation of Oregon — a solution that was only temporary. Led by missionaries, American settlers began to outnumber British settlers by the late 1830s. But Britain was not Mexico. Its powerful navy was still the largest in the world. Twice before had Americans taken up arms against their former colonizers at great expense to each side.