U.S. territorial expansion refers to the process by which the United States increased its land holdings from the original thirteen colonies to the vast territories it encompasses today. This expansion occurred through various means, including purchases, treaties, and conflicts. Notable events include the Louisiana Purchase in 1803, which doubled the size of the nation, and the annexation of Texas in 1845. The concept was driven by the belief in Manifest Destiny, the idea that Americans were destined to expand across the continent. This expansion often led to the displacement of Native American tribes and conflicts with other nations, such as the Mexican-American War in 1846-1848, which resulted in significant territorial gains for the U.S.