Complex sentence: There, Kilcullen served as the Senior Counterinsurgency Advisor until 2008 and was responsible for planning and executing counterinsurgency strategy and operations and was a principal architect the Joint Campaign Plan which drove the Iraq War 2007 Troop Surge.
Split into simpler sentences: There, Kilcullen served as the Senior Counterinsurgency Advisor until 2008 and was responsible for planning and executing counterinsurgency strategy and operations. He was a principal architect of the Joint Campaign Plan which guided the Iraq War 2007 Troop Surge.