I worry that we, as mothers, demonise the things that boys are interested in, just because 'we don't like them' (paraphrased).
Wars are a fact of life, they always have been, and save for some cataclysimic change in the world they always will be.
You, of course, are entitled to hold your pacifist views.
However, as long as wars feature in our lives, then we will always need brave men and women, to be prepared to fight for those freedoms which we hold dear.
Wars are fascinating for young boys, why should we censure their interests?
My DS 6yo, loves drawing, but the vast majority of his drawings are of battle scenes - we joke in our house that as long as we have a stock of sludge green, grey and orange pencils in the house, DS can be kept occupied for hours
It is for him, just a form of fantasy, like star wars I suppose. He does ask about real wars and in talking about them, we do make it clear that war is an evil, dangerous, horrible thing, anything that kills people is. That is the time to try and balance the view he has of wars being 'exciting'!
FWIW in your position, I would sit down and do the painting of these fantasy figures, and use it as an opportunity to discuss your views.
I think it is best to make sure he understands that these are your views and why they are important to you, you can also discuss that other people have different views - a lesson in tolerance if nothing else