First things first let's start with their mission, to take back the great machine, their holy object. The only times the fallen have been able to pose a direct and immediate threat to the city is when they're united with one purpose, to once more lay claim to their great machine. In real life, Christian kingdoms weren't united either, but were often at each others throats, that is until a crusade was called, uniting them for a time. Many of the Christian kingdoms were also remnants of the Roman Empire, like how the fallen are remnants of their once great civilization. I could go more in depth linking specific fallen attacks on the city to different crusades, but I think this next part is convincing enough. Who is the leader of the iron banner, and the bane of the fallen's last greatest attack on the city, lord Saladin. In real life, who lead the fight against the crusader states after the crusaders occupied the holy lands, their most successful crusade, Salah ad-Din Yusuf, also referred to as Saladin. Now the fallen have retreated, their banners are burnt and their houses are no more, but they are not gone, only hidden, once more united under their new leader, their Charlemagne. Does anyone remember from the early Destiny concept art where thorn was supposed to come from, none other than Charlemagne's vault.