Complex sentence: He defeated Severus' legate, Virius Lupus, and was able to lay claim to the military resources of Gaul, but although he made Lugdunum the headquarters of his forces, he was unable to win the allegiance of the Rhine legions.
Split into simpler sentences: He defeated Severus' legate, Virius Lupus, and was able to lay claim to the military resources of Gaul. And although he made Lugdunum the headquarters of his forces, he was unable to win the allegiance of the Rhine legions.