Rome wasn't a world power anymore and nothing had replaced it yet in Europe. Because of that, if you look at the 7th century on world scale, it is surprising for modern westerners how few European conflicts are worth mentioning. See for example http://en.m.wikipedia.org/wiki/Category:7th-century_conflict..., http://www.fsmitha.com/time/ce07.htm

The earlier Romans and later Europeans, starting with Charlemagne and the Vikings in the 8th century, make up for that, though.

Muslims were cruel in those times, but I don't think they were anything special (read that section about Constantinople in 602 in that first link, and think of Rome, the crusades, and the colonization of Africa, centuries later). They just happened to have the overhand for a while.