I am so glad you brought us back to the Roman Army of Late Antiquity, the original crusaders of Christendom. A lot of what Catholicism became echoed the old pagan order, so fell afoul of the architects of the New IsrŠ°el in These Goings Down of the Sun. The idea of Rome, all the way down to the Holy Roman Empire, which was none of those three things, was the bones upon which the flesh of early Christianity was hung...
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My all time favorite crusader type episode is from the Byzantine-Sassanid war, where Emperor Herakleus got the church to melt down their gold and sliver so he could raise a host and drive into the heart of Persia to reclaim the True Cross. Everyone forgets about this because the Muslims come in and trash the world a decade after but still was a thrilling tale.
What a savage war that was. There needs to be a movie.