The story behind ArmA 2 is both cohesive and coherent, making perfect sense each step of the way, but the most impressive thing about it, is its ability of adjusting to our decisions. Basically you are a highly trained US Marine named Cooper, sent along with the Razor team into the fictional state of Chernarus to support the democratic government in its struggle against the communists insurgents. The political context is complex enough to make you wonder about every action and frequently you'll be left out wondering if you're supporting the right side.

Since there are 5 different factions fighting for power, you'll be forced to take sides more then once and sometimes your actions can influence the fragile balance of power and lead to unexpected alliances. Of course the communists are essentially the bad guys, and during the campaign you'll encounter enough situations where the "reds" are guilty of war crimes, but the nationalist partisans are no saints either and there are few things they'll refrain. You are given the opportunity to decide if certain side missions are worth taking, but risking the lives of your teammates to aid civilians or unveil atrocities committed by one faction will unlock new features.

Furthermore there are multiple ways of tackling each problem, as you can either resort to strategic air strikes that will neutralize a target swiftly, but endanger the life of civilians or take chances and destroy the objective with a satchel charge.  Depending on your approach more missions become available and winning the hearts of the locals has certain advantages on the long run. Interesting enough you'll encounter fewer civilians then expected even for a war-thorn area, so when you do run into one in distress, it's quite hard not to postpone the main mission and try helping him out.

In the early stages of the campaign you are only concerned of staying alive and lead your squad through the huge battleground, but as time goes by, more complex tasks are being handed out to you.  Since the AI is very competent and realism is the main attraction in ArmA 2, avoiding an untimely death is a huge challenge and the casual shoot and run approach is not going to work. Most of the time you'll be crawling through thick grass, or trying to find cover at all time, shooting just a few rounds in the critical moments.