Complex sentence: The Oklahoma Land Rush of 1889 was the first land rush into the Unassigned Lands and included all or part of the 2013 modern day Canadian, Cleveland, Kingfisher, Logan, Oklahoma, and Payne counties of the U.S. state of Oklahoma.
Split into simpler sentences: The Oklahoma Land Rush of 1889 was the first land rush into the Unassigned Lands. The area that was opened to settlement included all or part of the present-day Canadian, Cleveland, Kingfisher, Logan, Oklahoma, and Payne counties of the U.S. state of Oklahoma.