The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territories, 326 Indian reservations, and some minor possessions.At 3.8 million square miles (9.8 million square kilometers), it is the world’s third- or fourth-largest country by total area.[d] The United States shares significant land borders with Canada to the north and Mexico to the south as well as limited maritime borders with the Bahamas, Cuba, and Russia.With a population of more than 331 million people, it is the third most populous country in the world. The national capital is Washington, D.C., and the most populous city is New York City.
Paleo-Indians migrated from Siberia to the North American mainland at least 12,000 years ago, and European colonization began in the 16th century. The United States emerged from the thirteen British colonies established along the East Coast. Disputes with Great Britain over taxation and political representation led to the American Revolutionary War (1775–1783), which established the nation’s independence. In the late 18th century, the U.S. began expanding across North America, gradually obtaining new territories, sometimes through war, frequently displacing Native Americans, and admitting new states; by 1848, the United States spanned the continent. Slavery was legal in the southern United States until the second half of the 19th century, when the American Civil War led to its abolition. The Spanish–American War and World War I established the U.S. as a world power, a status confirmed by the outcome of World War II. During the Cold War, the United States fought the Korean War and the Vietnam War but avoided direct military conflict with the Soviet Union. The two superpowers competed in the Space Race, culminating in the 1969 spaceflight that first landed humans on the Moon. The Soviet Union’s dissolution in 1991 ended the Cold War, leaving the United States as the world’s sole superpower.
The United States is a federal republic and a representative democracy with three separate branches of government, including a bicameral legislature. It is a founding member of the United Nations, World Bank, International Monetary Fund, Organization of American States, NATO, and other international organizations. It is a permanent member of the United Nations Security Council. Considered a melting pot of cultures and ethnicities, its population has been profoundly shaped by centuries of immigration. The U.S. ranks high in international measures of economic freedom, quality of life, education, and human rights; it has low levels of perceived corruption. However, the country has been criticized for inequality related to race, wealth, and income; use of capital punishment; high incarceration rates; and lack of universal health care.
The United States is a highly developed country, accounts for approximately a quarter of global GDP, and is the world’s largest economy by GDP at market exchange rates. By value, the United States is the world’s largest importer and second-largest exporter of goods. Although its population is only 4.2% of the world’s total, it holds 29.4% of the total wealth in the world, the largest share held by any country. Making up more than a third of global military spending, it is the foremost military power in the world and a leading political, cultural, and scientific force internationally.
美利坚合众国（英语：United States of America），简称美国（U.S.A./U.S.），是下辖的50个州、华盛顿哥伦比亚特区、五个自治领土及外岛共同组成的联邦共和国。美国是当今世上唯一超级强国，全球的经济、军事、教育、科研、航天及医疗技术头号强国。美国的面积超过983万平方公里，正式为9,833,520平方公里，位居世界第三或第四；同时拥有接近3.3亿人口，为世界第三人口大国。
16世纪欧洲开始殖民北美。现今的合众国起始于东海岸的13个英属美洲殖民地。欧洲七年战争后，大不列颠王国与其殖民地之间的争议愈发剧烈，最终导致在1775年爆发美国革命。1776年7月4日，正与大不列颠进行独立战争的各殖民地派出代表，协同一致发表《独立宣言》。战争终止于1783年，大不列颠王国承认这13个北美殖民地脱离管辖而独立，与其签订《巴黎条约》。这场战争也是第一场成功脱离欧洲殖民帝国的独立战争。1781年，《邦联条例》在邦联13个构成州获得通过，共同组成了邦联议会，各实体组成了国家联盟，1787年《美利坚合众国宪法》完稿，合并一国后将邦联体制升级为联邦，“美利坚合众国”联邦政府随之成立。1791年，合称为权利法案的十条宪法修正案获得批准，担保了基本民权。
自19世纪起美国从独立的殖民地国家开始转向对外扩张，美国联邦政府通过屠杀、强行迁移原住民，征服及购买等方法大力扩张领土，期间陆续有许多新建立的国家申请加入联邦，随着逐渐不断地承认扩张领地为新州份，至1848年时美国疆域已横跨整个北美大陆。19世纪下半叶爆发的内战，期间南部退出联邦独立，但后来被美国以武力重新并吞，使曾经合法的奴隶制度得以终结。在19世纪末已将其领土延伸到太平洋的夏威夷。美国经济在工业革命的推动下，自那时起也开始蓬勃发展。随后美西战争的胜利，美国对外干涉的范围超过现在的美国本土，使美国势力进入加勒比海地区及太平洋西部；而参与第一次世界大战则奠定其作为一个全球性军事力量的基础。尽管在1930年代经历大萧条，美国在第二次世界大战获得胜利之后崛起成为超级大国。作为联合国安全理事会常任理事国，美国不仅是核武拥有国，亦是世界上第一个研发出核武器，并唯一一个曾将其投入实战的国家。战后美国与苏联进行了数十年冷战，顶峰时期的太空竞赛亦促使了人类第一次登月计划的成功。1991年末苏联解体，美国自此被广泛视为当今世界唯一的超级大国。
美国作为一个高度发达国家，是世界上最大的进口国及第二大的商品出口国，国内生产总值按国际汇率排名世界第一、而依购买力平均则位列第二。在国民平均薪资、人类发展指数、人均国内生产总值以及人均生产力等社会经济学表现指标上，美国均处于世界领先地位。美国经济已步入后工业时代，服务行业占据经济主导地位，位列世界第一。美国是世界上人口最多的发达国家，有着来自世界各地的大量移民，也是世界上民族和文化最多元的国家之一。美国地形与气候复杂多样，是多种野生动物的家园。
同时，其生产制造业规模也极为庞大，位居于世界第二。仅占据全世界4.4%人口的美国贡献了世界四分之一的国内生产总值和三分之一的全球军事开支，这使其在经济和军事上均处于全世界最重要的地位。美国在政治和文化上是一支世界显著并影响深远的力量，也在科学研究和技术创新上占据世界领导地位。美国也是联合国、世界银行、国际货币基金组织、美洲国家组织等组织的创始成员国以及总部所在地，在国际事务中占据重要话语权。
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