The PCs wake up in an abandoned Battalion Aid Station or in the smoking remains of a crashed medevac chopper. Their last memory is of a bright light emanating from the insurgents' complex and the eggheads opening the ANABASIS container. Whether it is a hospital orderly feasting on the open skull of a wounded soldier or a medevac pilot stumbling around with unnatural purpose after being decapitated by the chopper's main rotor, it is immediately obvious that the dead no longer stay dead and crave the life of the few still remaining alive. They seem to be temporarily immune to the virus, perhaps due to their proximity to ANABASIS when it was released. Once they flee their immediate surroundings and are able to raise communications, it becomes apparent that the Virus has reached everywhere in the world. Out of the 150,000 American troops deployed in Iraq on land and sea, there are perhaps 10,000 left alive. They are alone, surrounded by both the undead and whatever hostile insurgents who have not been "infected", and whatever contact they can make back home is sporadic, dying, and full of bad news. But there is an American destroyer waiting at Trabzon, free of infection and waiting to pick up any survivors that make the long march to the sea.
For those that didn't catch the references already, this a zombie game based on the march of the Ten Thousand, a group of Greek mercenaries that were left stranded in the middle of Persia in the 5th-century BC, having to march through hostile territory back to the sea after the Persian leader who hired them died in battle. Their story was chronicled by the Greek general Xenophon in his work Anabasis. I figure this to be an All Flesh Must Eaten game, but any zombie-friendly system would do. As for the source of the virus, it could've been an old Soviet experiment found by the Taliban and smuggled out of Afghanistan for maximum carnage in Iraq (or you could just ignore the Anabasis references and set the game in the Hindu Kush); or the ANABASIS package was the actual culprit, a designer virus targeting genetic markers American intelligence identifies with "insurgent ethnicities" as a final attempt to "win" the war, with the "insurgent stronghold" being GRAY FOX types impersonating terrorists to place the blame on Al-Qaeda; or a lost ritual to Ubbo-Sathla found by insurgents among the rediscovered annals of the Hashshashin... whatever floats your boat.
And as for why I'll never run it... well, some might consider it in bad taste as hundreds of thousands to over a million people have died in that war, and continue to die to this very day. I rather do, but more honestly, I'd never consider it because, if I were to run this at a convention, there's a decent chance there might be an Iraq War veteran or two at the table. I'd feel very uncomfortable, not so much for using their personal history as the backdrop for a silly zombie game, but moreso because everything I'd have to say about the environment, what kind of resources would be available, to just how characters might react in certain situations, would be obviously bullshit. I can roleplay an orc because it doesn't exist and exists only in fantasy. I can roleplay a Nazi because there are acceptable cinematic examples of Nazis and a screen of "fantasy" can be placed between the real Nazis and the face-melting Tohts (with that screen being moved as close to or as far from reality as your historical awareness and sensitivity allows). But there just aren't enough cinematic versions of the Iraq War to play against... yet.