Well power did really remain in rome through the massive influence of pope and the catholic church (many tribes and later monarch were catholiuc) remaining unchalledged til the 14th century. There was no really 'central miltary power' as the various tribes squabbled amoung themselves and eventually medevial monarchy with viking raping and pilaging in themiddle of this period. It was only in the 8th century when charlemagne (backed by the pope) emerged as the leader of the Holy roman empire that there was a strong source of power and influencethen of course the rest of europes nations sought to become more influencal and powerful in responce.

Hope that answers your question. :)