Complex sentence: The first arrivals were a force of one thousand men led by George Armstrong Custer to investigate claims that the area contained gold, eventhough it was known that the land was owned by the Sioux.
Split into simpler sentences: The first arrivals were a force of one thousand men led by George Armstrong Custer to investigate claims that the area contained gold. Eventhough it was known that the land was owned by the Sioux.