At age 49, James Polk was younger than any previous president when he entered the White House. A workaholic, America’s new chief executive set an ambitious agenda with four major goals: cut tariffs, reestablish an independent U.S. Treasury, secure the Oregon Territory and acquire the territories of California and New Mexico from Mexico. Polk eventually achieved all his goals. He was a champion of manifest destiny–the belief that the United States was fated to expand across the North American continent–and by the end of his four years in office, the nation extended, for the first time, from the Atlantic Ocean to the Pacific Ocean.
In 1845, the United States completed its annexation of Texas, which became the 28th state on December 29. This move led to a breakdown in diplomatic relations with Mexico (from which Texas had revolted in 1836). After the United States sent troops to a disputed border region around the Rio Grande River, the Mexican-American War (1846-48) broke out. The United States won the two-year battle, and as a result, Mexico relinquished its claims to Texas. It also recognized the Rio Grande as America’s southern border and, in exchange for $15 million, ceded the land that makes up all or parts of present-day California, Arizona, Colorado, Nevada, New Mexico, Utah and Wyoming. (Despite the U.S. victory, the war proved controversial and reignited the slavery extension debate that would ultimately result in the American Civil War in the 1860s.)
With the Oregon Treaty of 1846, Polk managed another significant land acquisition–this time without going to war–when his administration diplomatically settled a border dispute with the British and gained full control of the present-day states of Washington, Oregon and Idaho, as well as parts of Montana and Wyoming.
On the domestic front, Polk reduced tariffs in an effort to stimulate trade and created an independent U.S. Treasury. (Federal funds had previously been deposited in private or state banks.) Also during this time, the U.S. Naval Academy, Smithsonian Institution and Department of Interior were each established, and in addition to Texas, two more states–Iowa (1846) and Wisconsin (1848)–joined the Union.