Indian Territory, also known as Indian Country, Indian territory or the Indian territories, was the land set aside within the United States for the use of Native Americans ("Indians"). The general borders were set by the Indian Intercourse Act of 1834. It was more properly "Indian territory" (lower-case T) than "Indian Territory" (capital T) because the name referred to the unorganized lands set aside for Native Americans, as opposed to an organized territory meant for settlement by Easterners.
The Indian Territory had its roots in the British Royal Proclamation of 1763, which limited white settlement to crown lands east of the Appalachian Mountains. This was reduced under British administration and after the American Revolution further until it included only the lands west of the Mississippi River.
At the time of the American Revolution, many Native American tribes had long-standing relationships with the British, but a less developed relationship with the American rebels. After the defeat of the British, the Americans twice invaded the Ohio Country and were twice defeated. They finally defeated a Native American confederacy at the Battle of Fallen Timbers in 1794, imposing the unfavorable Treaty of Greenville, which ceded most of what is now Ohio, part of what is now Indiana, and the present day sites of Chicago and Detroit to the United States.
The Indian Territory served as the destination for the policy of Indian Removal, a policy pursued intermittently by American presidents early in the nineteenth century, but aggressively pursued by President Andrew Jackson after the passage of the Indian Removal Act of 1830. The Five Civilized Tribes in the south were the most prominent tribes displaced by the policy, a relocation that came to be known as the Trail of Tears. The trail ended in what is now Arkansas and Oklahoma, where there were already many American Indians living in the territory, as well as whites and escaped slaves. Other tribes, such as the Delaware, Cheyenne, and Apache were also forced to relocate to the Indian territory.
The Five Civilized Tribes set up towns such as Tulsa, Tahlequah, Muskogee, and other, which often became some of the larger towns in the state. They also brought their African slaves to Oklahoma, which added to the African-American population in the state.
In time the Indian Territory was gradually reduced to what is now Oklahoma and, with the organization of Oklahoma Territory, the eastern half of the state. The citizens of Indian Territory tried in 1905 to gain admission to the union as the State of Sequoyah but were rebuffed by Washington. With statehood in November 1907, Indian Territory was extinguished. Many American Indians continue to live in Oklahoma, especially in the eastern part.
The terms "Indian country" and "Indian territory" are often used interchangeably, although Indian territory usually has the more specific meaning outlined above — that is, the region in the West where American Indians were compelled to relocate in the nineteenth century.
Indian country is an expression generally used today to describe (collectively or individually) the many self-governing American Indian communities throughout the United States. This usage is reflected in many places, such as in the title of the American Indian newspaper Indian Country Today . 
In U.S. military slang, Indian country is any area where troops can expect to encounter armed opposition, a usage that became popular during the Vietnam War.