I was hoping someone could explain this to me. The scientists in the Lambda Complex teleported to Xen and received specimens, losing scientists in the process. However, it wasn't until Freeman's resonance cascade that the Xen finally attacked back? Did the Xen not have teleporting technology or something to attack Earth back?
shareI don't think the aliens from Xen had the capability of inter-dimensional travel, however, when the Resonance Cascade occurred it opened a portal big enough for the Nihilanth to hold open (Freeman had to kill it to stop the portal from being held open). This allowed the aliens to be randomly spawned in Black Mesa, the epicenter of the event. In the events between the two games, the Combine were able to harness the technology to open a portal of their own and invade earth. In the Black Mesa chapter of Half-Life 2, Dr. Mossman explains that the Combine have limited portal technology "Once they travel through they have to rely on local transportation". The Combine were able to call in reinforcements with the Dark Fusion reactors located in various parts of the world. Hope this helps.
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