China has, quite openly, been preparing for war with the U.S. for many years. Why the surprise about this?
Then there's..."Operation: Anchorage. Enter a military simulation and fight in one of the greatest battles of the Fallout universe the liberation of Anchorage, Alaska from its Chinese Communist invaders. Gain access to unique armor, weapons, and exotic gadgets while you build and command interactive Strike Teams to win the battle and defeat the Chinese base.
Story: The Brotherhood Outcasts are trying to acquire advanced military technology, and the only way to open the vault containing these relics is by completing a tactical simulation only you can enter. In Operation: Anchorage you will re-live the epic Battle of Anchorage from Fallout lore. Find your way into the simulation, stripped of resources, and survive within the rules set up by the simulations creators. The Chinese red army is everywhere, so secure the surrounding mountain side and fight your way into the Chinese base."
(h/t cadavor)