This reminds me of the time (possibly Ralph)Nader became shah of Iran and invaded india. The Mughal king was bragging about how he was the son of someone who was king and so on for several generations, and Nader shah responded by saying something along the lines of I am the son of the sword, and so on for more generations than you.

So yhea, ultimately legitimacy should come from the ability to raise an army and hold territory. But not entirely. there is always some sort of ritual involved, in order to provide some stability or institutional continuity.

But nader's regime didn't last long. It's institutions hadn't become strong enough to survive. They lacked a certain legitimacy in the minds of the subjects. And ironically so did the progenitor of the Mughal. It was amir Timur, acting as a regent of a descendant of gengis Khan. it was only after his death, and subsequent civil wars that Timur's descendents became kings in their own right even if their military power was much lesser than Timur's.

So there has to be some sort of continuity of ritual that forms the myth of the legitimacy of a given regime in the minds of it's people. And it's this the strength of this that prevents warlords from claiming kingship.

I think Japan is a good example, being isolated from the rest of the world, the warlords became shoguns, while the emperor remained a symbolic figure. Eventually the shogun also became a symbolic figure to the point were "Toy"O'Tommy Hideoshi wasn't even shogun but Taiko. But then his descendants were involved in the struggle to become shoguns, perhaps because of the interruption of ritual.