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 led to the American Revolutionary War (1775–1783), which established independence. In the late 18th century,with the help of the First Transcontinental Railroad the U.S. began vigorously expanding across North America, gradually acquiring new territories, oftentimes conquering and 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 and the Soviet Union engaged in various proxy wars, but avoided direct military conflict. They also competed in the Space Race, culminating in the 1969 spaceflight that first landed humans on the Moon. The Soviet Union's collapse in 1991 ended the Cold War and left 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 (OAS), NATO, and other international organizations. It is a permanent member of the United Nations Security Council. The U.S. ranks high in international measures of economic freedom, government corruption, quality of life, and quality of higher education. Despite income and wealth disparities, the United States continues to rank high in measures of socioeconomic performance. It is one of the most racially and ethnically diverse nations, and its population has been shaped through centuries of immigration. A highly developed country, the United States accounts for approximately a quarter of global gross domestic product (GDP) and is the world's largest economy by nominal GDP. By value, the United States is the world's largest importer and the second-largest exporter of goods. Although its population is only 4.3% of the world 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 is a leading political, cultural, and scientific force internationally.