Soon, many people from the United States started to move west. Now the United States was moving into Native American lands again. Starting around the 1860s, many Native American tribes fought back. These fights are now called the Indian Wars. The United States government reacted by forcing Native Americans to leave their lands again and move into concentration camps. The government called these camps "Indian reservations." (They were called "reservations" because some land had been put aside, or "reserved," for the Native Americans.) However, Native Americans were not allowed to leave their reservations. On some reservations, many people, especially children, died from hunger and sickness.