When did American expansionism begin?

American expansionism began in the early 19th century, gaining momentum after the Louisiana Purchase in 1803, which doubled the size of the United States. The idea of Manifest Destiny in the 1840s further fueled this expansionist sentiment, promoting the belief that Americans were destined to spread across the continent. This period saw significant territorial acquisitions, including Texas in 1845 and the Oregon Territory in 1846, culminating in the Mexican-American War (1846-1848), which resulted in further land gains.