That's a little deceiving. If we have 200,000 in theater, there are at least another 200,000, maybe as many as 400,000 who are training for the next deployment, or reconstituting from the previous one. Plus there are some numbers that are not in theater, but are nonetheless supporting the effort. Add to those the people who aren't really available for deployment. R&D, logistics types at depots, the whole uniformed staff at the Pentagon, and so forth. Probably more like 1.5 to 1.8 million available.
But all those could not be deployed to Northeast Asia. Ballistic missile crews and maintainers, in both the Air Force and Navy for example. OTOH, they might contribute to any war there if things went all to snot quickly.
Also remember that there are retirees like me that can be called back to back fill stateside training positions and many of us are still fully fit and capable of going to the front; have a lot of gray hair now though.