A few years ago, I started thinking about Constantine as the first Crusader. Of course, he was not a Crusader according to the standard definition—there were no grants of Papal indulgences or even a goal of Jerusalem; but in certain aspects I believe that it merits some thought. Constantine even framed his battle for Rome as a religious struggle. Rome was, and is, the principal Holy See and Constantine also fought against the pagan Maxentius.
So maybe Constantine could be viewed as a proto-Crusader…but it’s just an idea I mulled over during a seminar on Crusader history.