Okay kiddo, so a long time ago there were two countries called Poland and Lithuania. They were pretty close to each other and had been friends for a long time. But they thought it might be better if they joined together to become one big country!
So in 1569 they made an agreement called the Union of Lublin. This meant that they would share a king and work together as one big country called the Polish-Lithuanian Commonwealth.
Things went pretty well for a while. The Commonwealth was really powerful - it was one of the biggest and strongest countries in Europe! They had a big army, lots of money and lots of people.
But it wasn't always easy. The Commonwealth was made up of lots of different groups of people, like Poles, Lithuanians, Ukrainians, Jews and others. Sometimes they didn't all get along very well.
Then in the 1600s another big country nearby called Sweden decided to try and take over parts of the Commonwealth. There were also a lot of wars going on and lots of fighting between different groups of people inside the Commonwealth.
Finally, in 1648, there was a really big conflict called the Khmelnytsky Uprising. It was like a big fight between the Ukrainians and the Poles. Lots of people were hurt and it was really bad.
After that, the Commonwealth wasn't as strong as it used to be. It kept existing for a while, but eventually it fell apart and Poland and Lithuania became separate countries again.
But even though the Commonwealth didn't last forever, it was still really important in history. It helped bring together lots of different groups of people and showed that sometimes working together can make us stronger!