This movie is "believable". By that I mean it comes across as something that could really happen.
What I was sensitive to was how the kids involved came face to face with and had to deal with the realities of war. Comrades get killed. There are the retaliatory executions of civilians by the enemy occupiers. There is betrayal by one of their own, and the resulting justice on the betrayer. And there was the issue of how the violence of war tended at points to make the kids the same as the enemy in terms of the ruthlessness with which war is waged.
When I was in school, our education was not suddenly interrupted in a way that thrust us as 15 or 16 year old kids into fighting enemy troops. We got to go to our Sr. Prom, and walk across the stage and get our diploma and for the most part, go on with the rest of our lives. The kids in "Red Dawn" didn't get to do that. War stole it from them... They had to grow up before their time.
It was the loss of innocence and childhood that was the saddest thing about "Red Dawn".