The Battle of Halmyros was a big fight that happened a very long time ago, in the year 1311. It was between two big groups of people called the Angevins and the Catalan Company. They were fighting over who should have control of a place called Thessaly, which was like a big piece of land in Greece.
The Angevins were a group of people from France who wanted to take over Thessaly. The Catalan Company was a group of soldiers who had been hired by the Byzantine Empire to help them fight against some other people who wanted to take over their land.
At first, the battle was going really well for the Angevins. They had more soldiers and weapons, so they were able to push the Catalan Company back. But then something bad happened. The Angevins got too confident and started celebrating too early. They thought they had won the battle already, but they were wrong.
The Catalan Company saw their chance and attacked the Angevins from behind. This surprised the Angevins, and they weren't prepared for another fight. They tried to fight back, but it was too late. The Catalan Company was too strong, and they eventually won the battle.
After the battle was over, the Catalan Company took control of Thessaly. This was a big deal because it showed that they were really powerful and could win big fights even when they were outnumbered. The Battle of Halmyros was an important moment in history because it helped shape what Greece was like for many years to come.