empire especially within christian church caused problems ref name bauer bauer ''history medieval world'' pp ref visigoths invaded western roman empire although briefly forced back italy sacked city rome ref name bauer bauer ''history medieval world'' pp ref alans vandals suevi crossed gaul next three years spread across gaul crossed pyrenees mountains modern day spain ref name collins collins ''early medieval europe'' pp ref migration period began various peoples initially largely germanic peoples moved across europe franks alemanni
Applied science Military sciences Military history Medieval