Complex sentence: The first extent of dissent among the colonies was the Outer Reaches Rebellion, the military forces of the Terran Alliance was unable to put down the rebellion thus causing great upheaval within the alliance.
Split into simpler sentences: The first extent of dissent among the colonies was the Outer Reaches Rebellion. The military forces of the Terran Alliance were unable to put down the rebellion, thus causing great upheaval within the alliance.