My 1st grader son loves war "stuff". He plays at shooting, etc. His favorite type of history is military history. If that's the common pattern, it might be best to use war propaganda. It's not hard to find.
Kids find it easiest to relate to other kids, so I can start with this Disney video. It teaches the background in a way I think kids will be able to understand:
To illustrate the above, here's an example from Nazi Germany:
This is similar, except it comes from the USSR and is more defensive in emphasis:
Then, maybe put a US spin on thing by showing how we do propaganda: