Complex sentence: The tribes were relocated from their homes east of the Mississippi River during the series of removals, authorized by federal legislation, over several decades and moved to what was then called Indian Territory and is now the eastern portion of the state of Oklahoma.
Split into simpler sentences: The tribes were relocated from their homes east of the Mississippi River over several decades during the series of removals, authorized by federal legislation. They moved to what was then called Indian Territory and is now the eastern portion of the state of Oklahoma.