 Complete the following phrase:  However, the plans for retaking Africa had to be abandoned due to the immediate need to combat the invading Huns, who in 444 were united under their ambitious king Attila. Turning against their former ally, the Huns became a formidable threat to the Empire. Aetius transferred his forces to the Danube, though Attila concentrated on raiding the Eastern Roman provinces in the Balkans, providing temporary relief to the Western Empire. In 449, Attila received a message from Honoria, Valentinian IIIs sister, offering him half the western empire if he would rescue her from an unwanted marriage that her brother was forcing her into. With a pretext to invade the West, Attila secured peace with the Eastern court and crossed the Rhine in early 451. With Attila wreaking havoc in Gaul, Aetius gathered a coalition of Roman and Germanic forces, including Visigoths and Burgundians, and prevented the Huns from taking the city of Aurelianum, forcing them into retreat. At the 