Complex sentence: West Virginia became a state following the Wheeling Conventions of 1861, in which delegates from some Unionist counties of northwestern Virginia decided to break away from Virginia during the American Civil War, although they included many secessionist counties in the new state.
Split into simpler sentences: West Virginia became a state following the Wheeling Conventions of 1861, after the American Civil War had begun. Delegates from some Unionist counties of northwestern Virginia decided to break away from Virginia, although they included many secessionist counties in the new state.