When you're playing with your friends, sometimes you might want to work together and become one big team. That's kind of like what happened with the country of Bulgaria a long time ago.
A long time ago, there were two parts of Bulgaria that were separated from each other. One part was in the west and the other part was in the east. People in both parts spoke the same language and had similar cultures, but they were ruled by different kings and queens.
One day, some important people from each part of Bulgaria got together and they decided it would be better if they all worked together and became one big team. They wanted to feel like they were all a part of the same country instead of being separated.
So, they made a plan to join the two parts of Bulgaria together and become one unified country. They talked to the kings and queens of each part and convinced them to give up their power so they could have one big leader for all of Bulgaria.
It was kind of like putting together a puzzle. You have different pieces that are all separate, but when you put them together, they make one big picture. That's what happened when Bulgaria became unified. The two parts came together and made one big country with one leader.