Once upon a time, a long, long time ago, there were two big countries called Poland and Lithuania. They were right next to each other and they were friends. They decided to work together and become one big country called the Polish-Lithuanian Commonwealth.
Everything worked really well for a while and they were a very strong and powerful country. But then some bad things started happening. A group of people called the Cossacks started fighting against the Commonwealth and they were very good at it. They won a big battle in 1648 and it was very sad for the Commonwealth.
After that, lots of other countries tried to attack and take over the Commonwealth because they saw that they were having some trouble. But the people of the Commonwealth were strong and brave and they fought back really hard.
During this time, there were some very important people who were in charge of the country. One of them was King Jan III Sobieski and he was really good at leading the country during wartime. He helped the Commonwealth win some big battles and made them even stronger.
Even though there were some tough times, the people of the Commonwealth worked together to make sure that their country stayed strong. They had a really cool system called the "Golden Liberty" where everyone had a say in how the country was run, not just the king.
In the end, the Commonwealth lasted for a long time and they were a really important part of European history. They had some tough times but they always stayed strong and fought for what was right.