The First Battle of Callisto was an engagement between the United Nations and the Secessionist League in the first hours of the Jovian Moons Campaign. League forces ambushed UN peacekeepers and security forces on Callisto while a League freighter surprised a pair of UN Space Authority destroyers nearby. Though the attack on UN space forces failed, the Secessionist League ground offensive convinced the UN to temporarily withdraw its forces from Callisto and retreat to Europa.