Historian Walter McDougell points to manifest destiny [the belief that the expansion of the Americas was justified and inevitable] as a corollary of the Monroe Doctrine. For, he feels that while the Monroe Doctrine, which states that any efforts by Europe to colonize or interfere in any way with states in any part of the Americas, would be interpreted as an act of aggression, and there would be retaliation, it, thus, became necessary for territorial expansion, in order to enforce this Doctrine. During the Oregon boundary dispute with Britain manifest destiny played an important role since the Anglo-American convention of 1818 had as a provision the joint occupation of both Britain and the U.S. However, thousands of Americans migrated there through the Oregon Trail, forcing the British to recognize President Tyler's proposal that Oregon's boundary not be moved South along the Columbia River, giving Britain what is now the state of Washington, but, instead, along the 49th parallel. Advocates protested, demanding the annexation of the entire Oregon Line up to Alaska. This demand became a large issue in the Presidential campaign of Mr. James Polk.
Expansion also seemed inevitable given the personality of Americans: Like all the early immigrants, they, too, possessed adventurous spirits. In the East, there were disputed land claims which left some people bereft, but in the West, there were opportunities to own land free. There was also the lure of gold and silver. [Pike's Peak produced gold and there was the Comstock Lode of silver], adventure and opportunity, gold and silver to mine, jobs in building the railroad, managing general stores, the fur trade, and a place for former slaves to live without exploitation. Farming, in which the Homested Act provided 160 acres, and cattle-driving offered new lives for those who sought opportunities, as well. Of course, travel was easier as the steam engine was invented and people could travel by boat or train, thus speeding their journeys in answer to the lure of the West.