NATO Division Commander SPI torrent

This is probably the most in-depth modern warfare simulation that was made for civilian players. It covers the entire spectrum of C3 (command, control and communications), intelligence, logistics and more. Combat is handled as a function of movement: the more MPs used in the attack, the more combat bonuses received. Many innovations, yet playable.
Don't be deceived, however, this is a monster game in many senses of the word: it's not only big, but is intricately detailed. It's not for the faint of heart.