Election in fact was the only legal way of assuming the crown under German law: Even the German kings of the Mediavel dynasties officially all were elected, and be it in craddle at the father's swordpoint or by just a handfull of their own vassals. I take that as an echo of the Ancient days when "kings" not were rulers over their tribe but elected commanders of an army; that's in particular true for the western Germans.
I would see that as the way the Migration Periode "kingdoms" developed: as confederation of warbands, not tribes, electing the biggest fish as their king-commander. This does by itself lead to hereditary succession after some time as long as the most powerfull family is able to produce at least a half-way competent heir.
excuses, excuses.... ;)The two generals did squabble among themselves
Of course, the Roman army of that periode still was a militia force commanded by politians and burocracts; not much surprisingly that did often enough got a serious beating by all kinds of opponents. Hannibal's army BTW also had a large barbarian element with its Spanish cavalry and Celtic infantry.
On the other hand the professional Legions didn't fare seriously better when it came to fighting Germans. As long as we don't consider the Ancient Germans as somewhat superhuman, we must assume that an army emerging from the German woods was equipped much better than the popular image of them was (and is). And because we know that the common German was piss-poor and could at best dream of an iron helmet, to me, the most logical explanation would be that there was a considerable warrior class.
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