The Second Battle of Aachen was a battle during the First Viking Wars. Led by Atticus I of the Eastern Roman and Frankish Empires, the battle was an attempt to retake the city of Aachen. Following the Carolingian Victory, the Vikings were pushed back to Maastricht.
The Rhine Raids had all but crippled the Frankish Empire in the German areas. With the Frankish Navy faltering and the Frankish armies losing battle after battle, the late 840s were grim for the Franks. The Vikings relied on hit-and-run attacks to keep the Franks on the run. Cities, towns, and other settlements along the Rhine River were sacked by the Vikings. Utrecht and Cologne were among the prosperous cities that had been sacked by the Vikings.
In 849, the Vikings sent a large raiding party down the Meuse River. The Vikings were only able to take their ships so far. In the spring of 849, Maastricht was sacked by the Vikings, allowing them to easily reach both Liége and Aachen. Most of the Rhine Raids only resulted in the sacking of towns. Maastricht, on the other hand, was occupied. From here, the Vikings launched raids against several other settlements before, finally, a Danish raiding party led by Ragnar Lodbrok reached Aachen.
So began the First Battle of Aachen. The battle resulted in the destruction of large swaths of the city, including much of the Palace of Aachen. The Frankish Emperor, Lothair I, was killed when the Palace of Aachen collapsed. The Vikings occupied the city for three weeks until the Franks were able to retake it.
Atticus I, unwilling to allow Francia to suffer more defeats and potentially fracture, bypassed several traditions and customs to become the Frankish Emperor. Doing so was seen as a dishonor to Lothair I and his children, but Atticus disregarded the controversy for another time. Lending several fleets and Byzantine tagmata to the Franks, Atticus I and his forces surrounded the city before storming it.
Using the breach in the wall created by the Vikings during the First Battle of Aachen, the Franks were able to pour into the city. At the same time, the Byzantines were storming the southern gates of Aachen. Once the gates were breached, the Franks and Byzantines were able to merge their armies, storming the Vikings inside Aachen. The battle was much shorter than the First Battle of Aachen, though it resulted ultimately in a Frankish/Eastern Roman victory. The Carolingian armies initially assumed Ragnar Lodbrok had died in the battle, but his body couldn't be found. They later discovered that he had survived when Verdun was raided by Ragnar.
The Second Battle of Aachen was a Frankish victory. Unlike the First Battle of Aachen, this battle wasn't as damaging to the city. The southern gates had to be rebuilt due to the immense amount of damage dealt by the Byzantine siege engines. The battle was seen as the entry of the Byzantine forces, though they had participated in limited numbers. With Atticus I on their side, however, the Byzantine presence in the war was much higher from then on.