Well imagine you and your friend are playing with your toys and suddenly your friend wants to take your favorite toy without asking nicely. You feel angry because you love your toy and don't want to give it up. Now think of many countries around the world who had been playing nicely with each other, but then some countries wanted more power and land, and they started taking things without asking nicely, just like your friend.
This made many people and countries feel angry and scared because they didn't want to lose their freedom or loved ones. Some people believed that going to war was not the right thing to do because it would cause more harm than good, just like when you and your friend argue and end up getting hurt or breaking something.
These people who opposed the war believed that there were better ways to solve the problems and that all countries should work together to find a peaceful solution, like talking things out and compromising. They also believed that war caused too much destruction and hurt too many innocent people, just like when you see a fight on the playground and someone gets hurt.
In the end, the opposition to World War I wanted to prevent more harm and find peaceful solutions that would benefit everyone, just like you and your friend should learn to share and play nicely together.