To be fair, Nitro, some of the high-end "PC games" can crunch data as fast and in much the same way as military-grade sims of a couple of decades ago, and I know of at least one defense company that has used H3MilSim quite seriously. One advantage is that you can perform a reasonable campaign-level sim and avoid the huge secret-squirrel complications that arise at a national level.
It does all come down to open sources and some educated guesswork - but then, that's what you have on at least one side, unless you are very high in the secret squirrel tree.