The U.S. state of Kansas, located on the eastern edge of the Great Plains, was the home of nomadic Native American tribes who hunted the vast herds of bison (often called "buffalo"). The region was explored by Spanish conquistadores in the 16th century. It was later explored by French fur trappers who traded with the Native Americans. Most of Kansas became permanently part of the United States in the Louisiana Purchase of 1803. When the area was opened to settlement by the Kansas–Nebraska Act of 1854 it became a battlefield that helped cause the American Civil War. Settlers from North and South came in order to vote slavery down or up. The free state element prevailed.