A few years back I started playing Battlefield 2. Then, I found a free to download mod called Project Reality. I have not played another military style PC game since. The mod creators (Black Sand Studios) have make extensive changes to make the game as realistic as possible. Tactics and team play are #1 priority. Flanking, defending, calling in airstrikes, coordination of infantry and armor are all very important. In fact, the game is so demanding of teamwork, there are people who play a whole round doing nothing but flying transport choppers and running supply/logistical trucks. They often play the round never seeing an enemy or firing a shot but are super critical to a successful result.
Run and Gun and Lone Wolfing gets you killed quicky.
Check out some of the gameplay on You Tube. Search for Project Reality teamwork and you can see a lot of videos where people work together (Squaleader DBzao has some of the best)
There is even a manual to download before you play. It's pretty much required reading.
I am in NO WAY affiliated with EA Games or Black Sand Studios. I just really like this mod. It requires a lot of skill and mature play.