The United States embarked on a vigorous expansion across North America throughout the 19th century, displacing Native American tribes, acquiring new territories, and gradually admitting new states until it spanned the continent by 1848. During the second half of the 19th century, the American Civil War led to the end of legal slavery in the country. By the end of that century, the United States extended into the Pacific Ocean, and its economy, driven in large part by the Industrial Revolution, began to soar. 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 a global superpower, the first country to develop nuclear weapons, the only country to use them in warfare, and a permanent member of the United Nations Security Council. The end of the Cold War and the dissolution of the Soviet Union in 1991 left the United States as the world's sole superpower. The U. S. is a founding member of the United Nations, World Bank, International Monetary Fund, Organization of American States (OAS), and other international organizations.