As soon as the United States got possession of the land west of the Mississippi they began to explore it, and between 1804 and 1807 Lewis and Clarke had explored the whole basin of the Missouri, while Pike had investigated the country between the sources of the Mississippi and the Red River. We have already seen that Behring had carried over Russian investigation and dominion into Alaska, and it was in order to avoid her encroachments down towards the Californian coast that President Monroe put forth in 1823 the doctrine that no further colonisation of the Americas would be permitted by the United States. In this year Russia agreed to limit her claims to the country north of 54.40°. The States subsequently acquired California and other adjoining states during their war with Mexico in 1848, just before gold was discovered in the Sacramento valley. The land between California and Alaska was held in joint possession between Great Britain and the States, and was known as the Oregon Territory. Lewis and Clarke had explored the Columbia River, while Vancouver had much earlier examined the island which now bears his name, so that both countries appear to have some rights of discovery to the district. At one time the inhabitants of the States were inclined to claim all the country as far as the Russian boundary 54.40°, and a war-cry arose "54.40° or fight;" but in 1846 the territory was divided by the 49th parallel, and at this date we may say the partition of America was complete, and all that remained to be known of it was the ice-bound northern coast, over which so much heroic enterprise has been displayed.