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A firm supporter of Andrew Jackson, Polk was the last strong pre-Civil War president. Polk is noted for his foreign policy successes. He threatened war with Britain then backed away and split the ownership of the Northwest with Britain. He is even more famous for leading the successful Mexican–American War. He lowered the tariff and established a treasury system that lasted until 1913. A dark horse candidate in 1844, he was the first president to retire after a single term without seeking re-election. He died of cholera three months after his term ended.
As a Democrat committed to geographic expansion (or Manifest Destiny), he overrode Whig objections and was responsible for the second-largest expansion of the nation's territory. Polk secured the Oregon Territory (including Washington, Oregon and Idaho), amounting to about 285,000 square miles (738,000 km²), then purchased 525,000 square miles (1,360,000 km²) through the Treaty of Guadalupe Hidalgo that ended the Mexican–American War.
The expansion re-opened a furious debate over allowing slavery in the new territories. The controversy was inadequately arbitrated by the Compromise of 1850, and only found its ultimate resolution on the battlefields of the U.S. Civil War. Polk signed the Walker Tariff that brought an era of near free trade to the country until 1861. He oversaw the opening of the U.S. Naval Academy and the Smithsonian, the groundbreaking for the Washington Monument, and the issuance of the first postage stamps in the United States, introduced by his Postmaster General Cave Johnson. He was the first President of the United States to be photographed frequently while in office. Scholars have ranked him 8th to 12th on the list of greatest presidents for his ability to set an agenda and achieve all of it.