The expansion was magnified greatly by the Louisiana Purchase, when President Jefferson bought a large area of land west of the Mississippi river from Spain, the purchase added large amounts of futile, unsettled land to the United States. This purchase was made possible by the exuberant democratic belief in the Manifest Destiny.
The Manifest Destiny was a belief that Americans were meant to explore the western territories and settle the entire continent this, was strongly believed by most democrats and was a large political controversy. This, as well as making the Louisiana Purchase possible, made it so that President Polk could start the Mexican War.
The Mexican War was a war between America and Mexico during which the U.S. acquired large amounts of land formerly owned by Mexico. The war started in 1846 and ended in 1848. The war was prompted by the Mexicans invading the U.S. annex of Texas which even after the Texas Revolution in 1836 Mexico claimed as theirs. The war ended informally in the fall of 1847 when an American army captured Mexico City and formally in 1848 with the signing of the Treaty of Guadalupe Hidalgo.
Lewis and Clark also made a significant impact on the westward expansion by exploring enormous amounts of land, from the Mississippi to the Pacific Ocean. They were sent out by President Jefferson to find a water route across North America and, with the help of an Indian named Sacajawea, they tried to do exactly that and although they couldn’t find any all water routes...