The subject of war is difficult for adults to talk about let alone to explain to a child. However, it is a valid subject for the state of our world right now.
Start with talking about bullies in school. There are several steps that you take to deal with a bully. First, you can try to avoid them but that often causes them to get bolder in their actions against their target. Next, you can go to the teacher or principal and ask for protection. However, bullying children often have bullying parents that frighten the school officials into passive actions with the bully child. There comes a time where you have to make a decision between living with the torment or putting the bully in his place. I realize many mothers don't think a child should ever fight. But if a child is not taught to take a strong stand against injustice and evil then the bullies will rule the world. At this stage taking classes in self-defense methods is not only appropriate it is necessary for the safety of your child. At this point your child is taught not to be the aggressor but to stand firmly for what is just. You, as the parent, should have a talk with the principle explaining the bullying situation and that your child will not be the aggressor but has been taught to defend himself or others who are being harmed. Make it clear that if this occurs you do not expect that the principal will punish your child for doing what is right. In the same way leaders of nations can become bullies. Other nations try to give some disciplines to the bully nations to get them to choose better. They take away what the nation can buy. This is called sanctions. They send diplomats to talk to the bully leaders to see if they can settle things. But if they keep threatening or attacking other nations anyway then they have to be stopped with a fight which is called war between nations. The good nations should never be the aggressor and never start the war. But they must stand strongly against a nation that is hurting other nations or everybody will be very hurt. This is why a war begins.
Just as a naughty child needs to be stopped in their naughtiness so everyone is safe at school, so a naughty bully nation has to be stopped. We hope in both cases that the naughtiness will stop with the first efforts. But if not then there will be a battle to stop the bullying so everyone in the world can be safe from the bullying. Our nation is trained for self-defense if need be. We hope it will not be needed but we will stand firmly against a bully nation if it is necessary.