Complex sentence: Consequently, in the 730s, Arab raids were mostly limited to the immediate frontier regions and their successes became fewer, until in 740 the Byzantines inflicted a heavy defeat against the largest invasion force assembled after 718 at the Battle of Akroinon.
Split into simpler sentences: Consequently, in the 730s, Arab raids were mostly limited to the immediate frontier regions and their successes became fewer. By 740, when the Umayyads assembled the largest invasion force fielded after 718, the Byzantines had recovered enough to inflict a heavy defeat against them at the Battle of Akroinon.