Background
After the death of Odoacer, no Roman Emperor tried to intervene in the affairs of the newly created Germanic-Roman entities in the West. Justinian and his heirs prefered to invest their forces in the Balkans, keeping Avars, Slavs and Bulgars far from the Danubian area and forged friendly relations with some of their Germanic neighbours, like the Gepids and the Ostrogoths, which gradually embraced the Catholic faith and abandoned the Arianism.
However, the hostilities with the Sassanians in the East forced the Byzantines to launch exhausting campaigns against them at the beginning of the 7th century. The Roman Empire managed to conquer most of Mesopotamia, pushing the Sassanians to retreat to the Iranian plateau; but this effort, along with the increasing internal fights due to religious feuds (specially between Nicene and Monophysite Chrsitians), finally weakened the Byzantine positions in the East.
The rise of the Arab Caliphate in the 630s crushed the Byzantine dominion in the Middle East and Egypt. The Sassanians repealed the advance of them into the Iranian plateau, so the Arabs concentrated all their forces in the invasion of the Byzantine lands: Mesopotamia first, then Syria, Palestine, Egypt and Cirenaica.
By the end of the 640s, the Arabs had conquered all Armenia and reached the Caucasus. After that, they advanced unopposed towards central Anatolia, conquering the city of Iconium in 648. The Romans panicked, fearing that an eventual fall of Constantinople was approaching, and begged their Germanic neighbours for helping them.
The Ostrogothic Kingdom, by 650, was the most powerful Germanic-Roman kingdom in the West, only challenged sometimes by the Franks. In the previous century, the Ostrogoths had imposed the independence of Burgundy to the Franks as they wanted to establish a neutral buffer state between them; they had also defeated the Vandals, invading Corsica, Sardinia and Sicily and reducing the North African kingdom to a sort of vassal state.
The Ostrogoths agreed on helping Byzantium as they also feared the increasing presence of Arab ships in the Sicilian waters; the Vandals were also convinced by their dominant neighbours and by the fact that they also feared an advance of the Arabs towards the Vandalic region of Tripolitania. A third ally was the Gepid Kingdom, which the Byzantines had assisted many times against their hostile neighbours, the Lombard Kingdom.
In the previous decades, the Lombards had expanded his Kingdom through most of Central Europe (Silesia, Bohemia, Moravia...), expelling many Slavic tribes from their settlements. Unlike the Goths, Franks, Vandals and Gepids, they had not embraced the Catholic faith yet.
Map of Europe in 650 AD