My Dd is 6 and her paternal grandfather was a casulty of the Iraq war so I have had to explain war to her. She knows that governments are kinda like headteachers who decide what the whole country is going to do. Sometimes goverments think that another goverment is making bad rules so they send soliders to protect people from bad rules. Its really hard one to explain as I don't want her seeing things in terms of goodies Vs badies as it is rarely that black and white.
With time i found drawing out a physical timelime including reference points like great grannys bday, grannys bday etc helped as she knows how old they are.