Before The Seven Hour War...
After Gordon spoke to G-MAN, on Xen, at the end of Half-Life, he would have a "vision" of a huge portal / wormhole and can see the Earth and its cities ; The future direction of the Xen creatures... He stays there, standing, knowing that there is nothing he can do. - "A Battle You Have No Chance Of Winning"
PS: In the lower left and right, we can see that the "cities" merge with the rocks of Xen. A vision provoked by the G-Man.
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