Borders change over time. Sometimes the people in one region take over another area through violence. Other times, land is traded or sold peacefully. Many times, land is parceled out after a war through international agreements.
Sometimes, borders fall along natural boundaries like rivers or mountain ranges. For example, the boundary between France and Spain follows the crest of the Pyrenees mountains. For part of its length, the boundary between the United States and Mexico follows a river called the Rio Grande. The borders of four countries divide Africa’s Lake Chad: Niger, Chad, Cameroon, and Nigeria.
Borders—particularly national borders—affect travel and migration. People can usually move freely within their own country’s borders, but may not be allowed to cross into a neighboring country.
When neighboring countries have similar wealth and political systems, their borders may be open and undefended. For example, citizens of the 27-country European Union may travel freely among any of the member states. Only five EU members—Bulgaria, Cyprus, Ireland, Romania, and the United Kingdom—require travelers from other EU states to present a passport or ID card at the border.
