From the beginning of English colonization of America, one of its characteristic features was the constant westward shift of the frontier. When the frontiersmen reached the mid-eighteenth century almost within sight of Spanish and French colonies and their advance faced the threat of being halted, the Atlantic world underwent cardinal changes, and new horizons opened up before British settlers. The War of Independence only confirmed this reality, and in 1787, Thomas Jefferson could articulate the idea of creating a continental federation. One of the conditions for realization was finding a route across North America in the area of the Mississippi and Missouri rivers, and it was Jefferson who, after the purchase of Louisiana, sent the Lewis and Clark expedition westward in 1804 with this goal. During their two-year journey, the expedition successfully fulfilled its task and opened the northern plains, the Rocky Mountains, and California with Oregon first to fur trappers and later to standard colonization.
