Yes, they caused, or contributed greatly to,  the fall of the mighty Roman Empire (yeah.  mighty egotistical  :-), but they were pretty smart going about it.  And who is this Roman Zeno dude?  He's seemed pretty dumb, and Theodoric took advantage of it and eventually ruled Italy (thanks Zeno)Also, I did not realize the contribution the Germans had to the VAST expansion of Chritianity.

Depending on which tribe(s) you're referring to, I think it was protecting themselves at first which led conflict with Rome, and later barbarians in the 5th century saw large cracks developing in the Roman strength and decided to take advantage.