Complex sentence: The Triennial Baptists opposed the Indian Removal Act, passed by the Democratic-Republican-controlled Congress in 1830, which relocated several Native American tribes from east of the Mississippi River to the new Indian Territory (Oklahoma) (Oxford 378-9).
Split into simpler sentences: The Triennial Baptists opposed the Indian Removal Act, passed by the Democratic-Republican-controlled Congress in 1830. It relocated several Native American tribes from their historic territories east of the Mississippi River to the newly designated Indian Territory (Oklahoma) (Oxford 378-9).