Arma 3 came out a while ago, but without any single-player campaign. Bohemia Interactive decided to release the campaign after launch as free downloadable content. The first of the three planned campaign DLCs is out now, and it's already installed for you through the magic of Steam if you have Arma 3 installed.
The campaign story is filled with exciting military things like NATO, civil war, flashpoints, task forces, and a soldier named Ben Kerry. Even if the campaign doesn't sound exciting to you, it adds more content to the multiplayer and mission creator tools.