system
Given the following post, respond as if you're commenting on Reddit:
human
[Half Life] What exactly are the combine?

I'm not too brushed up on my half life lore but I did love to play back in the day. What's the lore of the combine? Why are they enslaving earth? What do they have to do with Xen?
gpt
The Combine is an inter-dimensional empire that invades planets to harvest them for resources for an as-yet unknown purpose. It's suspected that they just use said resources to perpetuate their own cycle of invasion and conquest, but it's still somewhat unclear. What we do know is that they have access to sufficient technologies and assets totally conquer Earth within lest than a day, the focal point of which was the 7 Hour War.

Xen was a haven from the Combine for the refugees of multiple conquered worlds, existing in the space between dimensions. When the calamity that befell Black Mesa breached into this inter-dimensional space, it alerted the Combine to the presence of Earth, directly resulting in their invasion.