Who do historians think the man in the iron mask was? Is it known whether he was a political prisoner or something else, or why he was subjected to this punishment?

by td4999
mikedash

There is a good deal of room for fresh responses to this question. But I briefly summarised the main controversies in an earlier thread here. The short answer is that we have no certainties about this at all, only a lot of supposition and a certain amount of reasonable deduction.

Even the discovery of fresh contemporary documentation in the last couple of decades has not really moved us any further forward.