When Miraz had a son, he wanted to kill Caspian for being the heir to the throne and his twin sister, Cassiopeia. That's why the two of them ran away. Caspian had a horn that had the magical property of calling for help in times of extreme need. When Caspian and Cassiopeia were in danger, he used the horn to summon Pevensie to Narnia. The Pevensie go to the Narnian camp, where at this time Caspian, Cassiopeia and his allies are discussing further actions. Suddenly Peter and Caspian come face to face. At this point, tension first arises between them and they fight with swords. Peter pinned Caspian against a tree, applying a cold blade near his neck, but he also felt the sword behind him.

“I advise you to let go of the sword.” Cassiopeia warned

“Peter, stop!” said Susan and Peter pulled away from Caspian

“Peter? King Peter?“ Caspian was surprised

“I thought you’d be.. older“ Cassiopeia commented

“If you want, we can come back in a few years.”
teased Peter

“No, no!” Caspian looked at his sister warningly

Peter looked at Cassiopeia. She was beautiful. Very much so. And Peter froze slightly, looking at her.