amendments guaranteeing many fundamental civil rights and freedoms, was ratified in 1791.
Through the 19th century, the United States embarked on a vigorous program of expansion across North America. It displaced native tribes, acquiring the Louisiana territory from France and Florida from Spain. It annexed the Republic of Texas in 1845, leading to war in which it conquered half of Mexico. It purchased Alaska from Russia in 1867. During the early territorial expansion, significant disputes between the agrarian slave-holding South and free-soil industrial North led to the American Civil War. The North's victory reestablished the Union, and led to the ratification of the Thirteenth Amendment to the United States Constitution, ending legalized slavery in the United States. The Plains Indian Wars relocated remaining tribes onto confined reservations, a Congressional Resolution annexed the Republic of Hawaii, then the treaty ending the Spanish-American War ceded Puerto Rico and Guam. By the end of the nineteenth century, its national economy was the world's largest.
The Spanish–American War and World War I confirmed the country's status as a global military power. The United States emerged from World War II as the first country with nuclear weapons and a permanent member of the United Nations Security Council. The end of the Cold War and the dissolution of the Soviet Union left the United States as the sole superpower. The U.S. economy is the world's largest national economy, with an estimated 2011 GDP of $15.1 trillion (22% of nominal global GDP and over 19% of global GDP at purchasing-power parity). Per capita income is the world's sixth-highest. The country accounts for 41% of global military spending, and is a leading economic, political, and cultural force in the world