This happens to my my own game of choice, the idea is that the ISA and the Helghast troops are fighting eachother over control of each others territory. The first one, KillZone, is that ISA are fighting of the Helghast on Vetka (a colony in the sky).
The second game the ISA are taking control of Helghan. The Evil dictator of Helghan, Visari, initiated both attacks (from game one and two).
I found these shooters to be alot of fun. (Only availible on PS2 and 3 systems)