It probably started around the cod 4 timeframe. Every game after modern warfare 1 was very pro America and when the Russians were depicted they were shown as vengeful (understandable from our perspective but you can tell the message was intended to be a negative one) or as villains. Also cod 5 (World at War) specifically was about the Marine raiders, the Special Operations component of the marine corps. They were disbanded after WW2 but were revived around the time of World at Wars release (within a couple years) so it was pretty obviously propaganda to join the marines.