Which president was known for the aggressive expansionist policies during the 1840s?

James K. Polk is recognized for his aggressive expansionist policies during the 1840s, a period marked by significant territorial growth for the United States. Under his administration, the U.S. pursued a doctrine known as Manifest Destiny, which was centered on the belief that Americans were destined to expand across the North American continent.

Polk's presidency saw the annexation of Texas in 1845, which was a contentious move that heightened tensions with Mexico. This led to the Mexican-American War from 1846 to 1848. Polk aimed to acquire California and New Mexico, which resulted in further military engagement. The war concluded with the Treaty of Guadalupe Hidalgo, through which the U.S. gained a vast expanse of territory, including present-day California, Arizona, New Mexico, Nevada, Utah, and portions of Colorado and Wyoming.

Additionally, Polk's administration also secured the Oregon Territory through negotiation with Great Britain, thereby further expanding the nation's reach. His commitment to territorial expansion fundamentally shaped the future of the United States and laid the groundwork for its continental ambitions. This era under his leadership is often viewed as one of the most significant in terms of American territorial expansion.