Hilarious antisemitic conspiracy theories here:
Al-Andalus was destroyed more by ethnic infighting between Arabs, Berbers and Muladíes, along with the more general issue of cousin-marriage-generated kin groups that hindered nation-building throughout the Arab world (and which prompted most Muslim dynasties from the Abbasids onwards to turn to slave soldiers, as any other military system would be colonized by said kin groups).
The Ottomans declined and fell because the population their territory could support was limited and they ended up too badly outnumbered (by Russia especially).
It was the Great Depression that allowed the Nazis to take over Germany.