Combat Mission: Shock Force Marines is the best modern warfare simulation you can get without a Top Secret clearance.
Games like Company of Heroes are great and fun, but leave me with a feeling that it's just a game. Marines has more of a realistic feel to it. It was definitely designed with realism being it's top priority.
The Game: US vs Syria is the stage for the game. Command up to a Battalion (more?) of Marines. Turn based or wego. Attack, defend, secure objectives, etc. Infantry, armor, artillery and air support, and specialized weapons teams are yours to lead to victory. Many user made scenarios and campaigns give you endless possibilities.
Longevity: There are tons of great mods, regular patches and updates, and a map editor and scenario designer. You don't just run through this game in a few hours.
Support is top notch. They actually get back to you as soon as possible.
With a British mod and a ww2 mod in the future, it will only get better.