The Great Northern War was a big fight that happened a long time ago between some countries in Europe. It was like a big game of capture the flag, but with armies and lots of fighting.
One of the big countries was called Sweden, and they were really good at fighting. But then some other countries like Russia, Denmark-Norway, and Poland-Lithuania all teamed up to take on Sweden and try to win.
The fighting went on for a really long time, almost 21 years. It was really hard on everyone involved, with lots of people getting hurt and lots of money being spent on the fighting.
In the end, Sweden lost and had to give up some of its land to the other countries. It was a really big deal in the history of Europe, and it changed the balance of power in the region for years to come.