As the world's fourth-largest country, the United States is made up of many landforms and climate types. Geographers divide the United States into five main physical regions: the Coastal Plains, the Appalachian Mountains, the Interior Plains, the Mountains and Basins, and the Pacific Coast.
With its wealth of resources and hardworking, inventive people, the United States has built the world's largest economy. Each of the country's five economic regions is known for its distinctive resources, goods, and services. In the twenty-first century Americans must handle the challenges of pollution, new technology, and trade.
The United States is sometimes called a "nation of immigrants." Since the earliest people crossed the land bridge from Asia, Americans have come from every part of the world. After winning their independence from Great Britain in 1781, early leaders created a representative democracy. By the second half of the twentieth century the United States had become a world leader. America's culture reflects its diverse heritage.