You know, I think I could.
Firstly, all the soldiers I've killed in-game - Russian, German, Chinese, Korean, American - all start to blend together after a while. They're just grunts in uniform.
Secondly, I come from Australia. The idea of an Australia so large and powerful that it's the major bad guy in a war is kind of awesome.
It goes without saying it'd have to be a good game, like Modern Warfare. I wouldn't play something purely so that I'd get to shoot my countrymen.
Frankly, I think it's past due someone made a game like that. As in, a major title.