Starsiege. No, not tribes. Starsiege, third game in the earthsiege series. Tribes was actually a spin off in the same universe that got much more popular (with good reason, tribes was awesome).

Starsiege was a pc game that came in a bigass box, complete with multiple books filled with lore from the game’s universe. It wasn’t as well received as mech warrior (another mech sim), so it didn’t have a big community. But those of us that played it loved it to death. I think you can still get the game running if you don’t mind fucking with a bunch of sketchy third party patches.