Okay kiddo, so imagine you are playing with your toys and you have a lot of toys but not enough space to play with them. That is kind of what happened with Japan a long time ago. They wanted more land to play with, but they didn't have enough land in Japan.
So, they decided to go and look for more land to play with in a place called Manchuria, which is in China. They went to Manchuria because it was close to Japan and they wanted a place with good resources like land, water, and minerals, so they can build more things and have more fun.
Now, when the Japanese got to Manchuria, they started to build homes and farms just like they did in Japan. They also started to run some businesses there. They really liked living in Manchuria because they had more space to play with and it was easier to get the resources they needed.
But, even though the Japanese enjoyed living there, the Chinese people who already lived in Manchuria didn't like it. They felt like the Japanese were taking over their home and they didn't want them there. This caused some problems between the Japanese and the Chinese people, and there were some conflicts between them.
So, in summary, a long time ago, the Japanese wanted more land to play with and they found it in Manchuria. They built homes and businesses there and enjoyed living there but some conflict arose between them and the Chinese people who already lived there.