Vandals at the gate
Rome suffered the same fate in a somewhat reverse way.
Rome was never conquered by invaders. As their legions became nation builders instead of soldiers, they intermarried, adopted local customs and religions and they became the very people that they had conquered.
In this case the conquered have come to Europe (the mountain has come to mohhammed) and instead of adding diversity and vitality they bring anti-modernity, fundamentalism and they may be sucking the life out of western civilization.