S.T.A.L.K.E.R.: Shadow of Chornobyl

You'll get this achievement as soon as you choose your side. It's either Duty or Freedom.

Duty is Warsaw Pact-aligned faction. They use Soviet designed weapons and are police of the Zone. As soon as you enter an ambush on your first visit in Army Warehouse, you can start their quest line and assault Freedom base. I don't know when the achievement should pop exactly as I went with Freedom.

Freedom are hippies of the Zone. Heavy drug and alcohol use are common theme of this faction. They are Nato-aligned and use western weapon designs. After the ambush you can approach their base and let Freedom know that Duty is staging an attack in nearby village and Lukash will give you mission to attack preemptively. As soon as you shoot and Duty turns hostile, you'll get your achievement.

You don't need to pick a side for this permanently. First, make a save where Voronin is and then kill all of them inside. I was curious earlier when I got this if it was possible this way. Sure enough, it is. Now, you can continue as a Loner, as things should be.