Complex sentence: The Harmar Campaign was an attempt by the United States, in the fall of 1790, to subdue Native Americans in the Northwest Territory who were seeking to expel American settlers they saw as interlopers in their territory.
Split into simpler sentences: The Harmar Campaign was an attempt by the United States to subdue Native Americans in the Northwest Territory in the Autumn of 1790. They were seeking to expel American settlers whom they considered interlopers in their territory.