You know, for a good idea of what they mean by 'a seemingly alien force', watch Flags of Our Fathers. During the invasion of Iwo Jima, the Japanese used the extensive netword of underground tunnels to wage war against the American invaders. They had the ability to appear even thousands of yards behind the frontline, deep into American-held territory. And they used this ability. Some Americans record that they woke up one morning with a dagger stuck in the ground beside their head, with a message written in eather poor English or Japanese (sometimes both). There are also accounts of soldiers seeing fellow men in neighboring foxholes suddenly get 'swallowed up', and when they ran over to investigate, not a trace of them could be found. While it is now known that those soldiers had probably had the incredible misfortune of digging a foxhole smack dap on one of the many tunnel entrances the Japanese had scattered throughout the island, you can imagine what kind of effect these sort of things would have had on the American troops. The enemy, which mostly remained unseen, would have appeared to be surreal...almost not of this world. Verge_6
That would be really nice if they manage to put that in the game and keep up that atmosphere, imagine walking with your squad, you are leader but when you look around you realise all of a sudden you are alone.