The seven hour war is an event in which a hostile alien force known as The Combine is invading earth. There are two categories of units. Synths or regular units. Synths are a collection of various species that are cloned cyborg and biomechanical aliens used by the Combine as war machines for their army. The synths are: Striders, Crab synths, Combine dropships, Combine gunships, Hunters, and super soldiers, and combine advisors. Regular units are standard units: (See definition)