Okay kiddo, so imagine there's a big playground where a few kids try to boss around all the other kids. That's kind of like what happened in the early 1900s with America and some other countries. America wanted to be the boss of other countries and tell them what to do.
But not everyone thought that was fair. A group of grown-ups got together and formed the All-America Anti-Imperialist League, which was like a club for people who didn't want America to be a bully. They thought that every country should be able to make their own choices and not have to listen to America all the time.
The members of this club wrote letters and articles to spread the word about their ideas, and they even protested when they thought America was doing something wrong. They wanted to help people understand that being a bully was not a good thing, and that everyone should be treated equally.
So, the All-America Anti-Imperialist League was a club of grown-ups who thought that America should not be a bully to other countries, and they worked hard to spread their message to others.