As different as each of them were, they all had one thing in common, at some point one side wanted more troops.
Most battles eventually come down to boots on the ground and rifles in the field. So when commanders are building their ranks it's often with professional soldiers who know how to fight, and get paid well to do it.
With more than 625,000 employees, this listed security giant is the second-largest private employer in the world (behind Wal-Mart). While some of its business is focused on routine bank, prison and airport security, G4S also plays an important role in crisis-zones right around the world.