Desensitisation and increased agression are things which have been attributed to games for a long time now with no real solid evidence besides 'it sounds about right', I'm skeptical about citing studies from either side of the debate for the main reason that they tend to either be politically motivated, have bad methadology or tend to come to their conslusions based more on assumption seeing as things such as aggression and temperment are high personal and subjective and cannot really be measured in a quantifiable or experimental fashion.
My opinion is that while violent or militaristic video games can have a certain level of influence it's no greater an impact than any other activity a person might do such as watching TV, watching the news, talking to friends and family or even just seeing billboards as you go about your day. All of these things influence us in a very minor way, this goes without saying, but to say that they have major impacts on our behavior seems very unlikely to me.
This model of violent images resulting in violent behavior is highly dependant on the consumers of a given media just being passive sponges who absorb anything they're shown and will just accept the messages (and more importantly will possess the understanding of these messages that the creators of a given media text intended us to have), all of these conditions are somewhat hard to back up though.
As for the idea of all viewers being passive, the fact that our collective understanding of the workings of the media and the use of subtext and symbolism shows that on the whole we all tend to be active participants, even in more 'passive' forms of media such as television, even though we don't directly impact the events on screen we do think over what we see and come to our own conclusion as to whether we agree or disagree with what a show tells us.
All media have messages in them (even the more simplistic games like Doom have the basic message of 'hell is bad, so is dying') and it is often very easy for us to not have the reaction that the creator of a film or game intended, for example, how many people do you know who think that the Dark Knight's Joker was right? Or who thought or Watchmen's Rorsharch (excuse my bad spelling) was an admirable or heroic character? Or sympathise and agree with the villains in video games? This again makes it harder for the 'corruption' of people by video games to be possible since this corruption effect would imply that there is a single message or view that the game is trying to manipulate us into holding and as a general rule this is just plainly impossible for all media forms outside of explicit propaganda (the way people think is too diverse and multi-lateral for such an effect to be created).
For the 'tl;dr' crowd, what I'm trying to say is yeah, some people probably did feel inclinded to join the army after playing CoD, but a lot more people didn't. Unless there's a trend where the majority of people who do something show the same behavior it's somewhat hard to prove that games are causing any noticable negative effects in people that wouldn't be caused by any other activity they could be doing, from what I've seen and read, such a trend simply doesn't exist outside of the rantings of the anti-gaming moral guardians.
