So, the US learns of the great resource and science potentials of the Zone in the STALKER video game series and they begin to make preparations to set themselves up there. Since there seems to be almost no official government forces or anything there at all, they don't encounter much political resistance and by the events of early Clear Sky, they begin to arrive in great numbers into the Zone.
What would be the implications of their arrival? Would they have better success than the rag-tag Ukrainian military that wander around there from time to time?
Let's say that their first objectives are to secure the Labs X-16 and X-18, secure the healing pond, and establish good presences at the Army Warehouses, the Dark Valley, Cordon, and most of the other most important areas. The US industry also demands a large amount of powerful artifacts to research and the US government/military is able to comply with them.