Who are the Combine?

In the beginning of the game, you are simply dropped into a world, where there are robot-like creatures attacking you. Who or what are the combine? Are they a part-robot, part-human race? Are they new aliens? Are they drones made to replicate an army, or police force?

Solution 1:

The Combine forces most frequently encountered in Half Life 2 are, or once were, humans. At one point in the game

you visit Nova Prospekt, where humans are converted into Combine Overwatch soldiers and special forces.

However, the Combine itself is more than just the military/police force you see in City 17 and the surrounding areas. It's

an interdimensional empire composed of many allied and assimilated species – hence the name "Combine". Humanity has become part of the Combine.