Which preposition to use with avars

in Occurrences 2%

After this nothing more was heard of the Avars in the Balkan peninsula, though their power was only finally crushed by Charlemagne in 799.

against Occurrences 1%

It is very doubtful whether the Bulgars did take part, as they are supposed to have done, in the ambitious but unsuccessful attack on Constantinople in 559 under Zabergan, chief of another Tartar tribe; but it is fairly certain that they did in the equally formidable but equally unsuccessful attacks by the Slavs and Avars against Salonika in 609 and Constantinople in 626.

Which preposition to use with  avars