Complex sentence: During the Syrian Civil War, multiple opposition and anti-Assad parties in the conflict have accused Assad of collusion with ISIS; several sources have claimed that ISIS prisoners were strategically released from Syrian prisons at the beginning of the Syrian Civil War in 2011.
Split into simpler sentences: During the Syrian Civil War, multiple opposition and anti-Assad parties in the conflict have accused Assad of collusion with ISIS to some degree. Several sources have claimed that ISIS prisoners were strategically released from Syrian prisons at the beginning of the Syrian Civil War in 2011.