Thomas Jefferson believed that the nation's future depended on its westward expansion. In 1803 the Louisiana Purchase took place, doubling the size of the country. By 1840 almost 7 million Americans had migrated westward in hopes of securing land and being prosperous. The belief that settlers were destined to expand to the west is often referred to as Manifest Destiny. Manifest Destiny was used to justify the war with Mexico in the 1840s which resulted in the acquisition of one million square miles of land previously owned by Mexico, enalrging the US even more. When the Civil War ended the US still faced the battle of conquering Western territory and this is how it happened.