The Combine are very similar to the Empire in Star Wars or a multi-nation dictatorship. They are never seen without their armored uniforms. The Combine are an empire spanning multiple planets that have taken to using the Xen to invade other worlds or dimensions to further their territory.  They are the driving force and are not allied with the Xen species such as the Headcrabs. While the Combine might use tactics that harness the feral capabilities of creatures such as Headcrabs or the power-generating qualities of a Lightning Dog. However, this is more like capturing lions and releasing them in a village. I currently have the theory that G-man and his employers seek to deal with the COmbine. Whether it is for benevolent reasons or if they wish to take over in the resulting power vacuum has yet to be confirmed.