James K. Polk (#11)

James Knox Polk (1795–1849) was the 11th President of the United States and arguably the most effective one-term president in American history. Before taking office, he announced four specific goals: acquire California from Mexico, settle the Oregon boundary dispute with Britain, lower tariffs, and establish an independent treasury. He achieved all four — then kept his promise not to seek re-election and died just 103 days after leaving office, exhausted by the work.

Polk's presidency fundamentally transformed the United States geographically. The Mexican-American War (1846–48), which he effectively provoked, resulted in the Treaty of Guadalupe Hidalgo — adding California, Nevada, Utah, Arizona, New Mexico, and parts of Colorado and Wyoming to the United States. The Oregon Treaty of 1846 extended the U.S. border to the 49th parallel, securing the Pacific Northwest. His critics — including a young Illinois congressman named Abraham Lincoln — challenged the justification for the Mexican war, but the territorial gains reshaped the continent permanently.