J.M.
One of my former students was shot and killed in Afghanistan this past year, and I was pretty upset. I thought it was best to be honest with my kids, so I told them that I was sad because a student of mine died very very far away from home. They asked how he died, and I said that someone shot him. The both (3 and 5 at the time) asked why someone did that. I explained that he was fighting in a war far away from home, and that sometimes grown-ups really can't work things out in a nice way, and that soldiers go in to fight each other, and sometimes people get shot and die and it's very very sad.
I'm pretty much a pacifist, so I talked up the angle of war being a sad thing and that I wish it didn't happen. That seemed to satisfy them.