Black Operations
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Black Ops
The Black Ops forces were compromised of highly trained assassin's dressed in a black, ninja-like uniform with masks to cover their faces.
They carried handguns and submachineguns as standard inventory and often flanked using grenades.
The Black Ops came in after the failure and retreat of the HECU forces.
Black Mesa Operation
The Black Ops operation at Black Mesa was a simple and straight-forward job, nuclear sabotage.
It involved infiltrating, planting the bomb and exfiltrating, though as they planted their package they were intercepted by Cpl. Adrian Shephard and the bomb deactivated - though the mysterious G-Man reactivated it for his own reasons and locked Shephard from getting back to the bomb.
What Became of Their Forces
There is no real information or evidence that the Black Ops forces still existed in the time span of Half-Life 2, Episode 1 or Episode 2, so it would seem they were completely wiped out by the Millitary, Combine or Aliens.