Complex sentence: Unionists took control of the state legislature when Kentucky's neutrality was breached, but Breckinridge fled behind Confederate battle lines where he was commissioned a brigadier general; he was then expelled from the Senate.
Split into simpler sentences: Unionists took control of the state legislature when Kentucky's neutrality was breached. After this occurred, Breckinridge fled behind Confederate battle lines where he was commissioned a brigadier general; he was then expelled from the Senate.