The alliance between aliens and combines...

[The Monkey]

Why are they together? Why are the vortiguans with you. Is this discusses after Ravenholm (where I am now) or did Mossman said that in the elevator? I never catched all she said there...

 

[clamothe]

I'm pretty sure that hasn't come up yet... I'm near the end of highway 17, after ravenholm, and I don't remember anything about it.. it's just one of those hl2 things... I bet we won't know alot until the end of hl2 or hl3...

 

[jb2002smg]

The Vortigaunts were a slave race freed by Gordon in Half-life 1. They are stranded on Earth, so instead of falling under Combine rule, they side with the humans.

 

[KagePrototype]

My take on things:

Combine invades Xen, takes over. Makes Nihilanth leader (much like they did with Breen). Vortigaunts are then enslaved. The Black mesa Incident occurs, and Nihilanth takes advantage and tries to invade Earth in revenge for what humans did to Xenians (see: Questionable Ethics chapter).

Freeman destroys Nihilanth, and sets Vortigaunts free somehow. Earth come in and manage to take over Xen. Combine fight back and reclaim it, and then take over Earth. Some vortigaunts get away and join the resistance (others do not, for example the janitor in the train station). Combine takes advantage of Xen invasion, by using Xen wildlife (barnacles, headcrabs) to draw people into the cities. Headcrabs are used as a cheap and effective way to get rid of large amounts of people, and barnacles are used to keep people thinking of escaping C-17 via canals.

As for posion and fast headcrabs, they are probably just a form of Xen wildlife you never saw in HL1 (like Ant Lions, keeps thing simple by saying they're from Xen as well).