Complex sentence: The Marias Massacre occurred in the context of massive white American westward expansion, and greatly increased tension between Indians and whites; the event which touched off the massacre involved a young Piegan Blackfoot named Owl Child.
Split into simpler sentences: The Marias Massacre occurred in the context of massive white American westward expansion. Amid this tension, the event which touched off the massacre involved a young Piegan Blackfoot named Owl Child.