The Sentinel droid-series (also known as the war-droid or military droid-series) was the only known series of android among the war machines used by the Galactic Republic and the Sith Empire in the Jedi Civil War. Variants included the Mark I, Mark II, Mark III, Mark IV, and Mark V, which were all outwardly identical. The HK-series was based on the Sentinel line in design and structure. Sentinel droids were equipped with blaster rifles and could benefit from personal energy shields, which, unlike spherical energy shields utilized by other models, allowed them to move while shielded.