Usually, you can play up to four against four, and in-game communication means alliances can be made or broken if you want to see game theory in action.
Chris
Warcraft 2 should be available
the original Command and Conquer was given away free and should be available somewhere just not on the Electronics Arts site anymore
This game is particularly effective : New Techno War (https://helvetia-games.ch/game-presentation/newtechnowar/). It was created by senior officers for senior officers. It's quick to learn. It allows you to practice attacking at section level. Many armies use it to train platoon leaders and analyze decision-making.