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The forces that defeated Earth's military forces all within just 7 hours. Obviously its not the overwatch patrols that we see in HL2, because those are just either humans in masks or human-based machines. Even the striders and gunships are supposed to be manned by humans (or what was once a human).
Perhaps advisor or advisor-like creatures is what took over the planet in such speed... using mind-attacks?
Whatever forces they used, I doubt Gordon has yet seen it... what do you think it was like?