The US ranks 3rd in population and size. It has different jurisdictions, including the District of Columbia and 50 states. Although different colonies had been established in the Americas, many of them failed due to diseases that killed many people, starvation as people exhausted their food, conflict with Native Americans, and war between the colonial powers. The US was formed by the 13 colonies that rebelled against colonialism. These colonies were divided into the Middle colonies, the Southern colonies, and New England. In 1803, the US acquired the whole of Louisiana for $15 million from France. The Louisiana Purchase paved the way for westward expansion, during which settlers began moving to the west in search of natural resources such as gold, coal, iron ore, and oil. These minerals played a critical role in shaping settlement in America. There was also adequate space and fertile land in the Great Plains and Midwest. However, the expansion was faced with challenges as there were no railroads, and maintaining contact with the east was difficult. Geographical features, especially the Rocky Mountains and the desert in the Southwest, also made it difficult for people to travel. The land also required irrigation since it was dry.