Latest PoD for Constantinople to fall to non-Muslims

Title is pretty much self-explanatory. How late could it have been plausible for a non-Christian, non-Muslim power to take Constantinople and hold it for a while afterwards?
(If you don't think it's possible at all, I suppose you can always answer 622 CE...)
 
At first, I was looking at just the title, which just mentioned muslims, and thought, "April 1915 or perhaps later," but for a non-christian power...not so sure. By chrisian power, do you mean one that's officially christian, or just one with a majority?

If Turkey ended up, somehow, as an Axis power, and D-Day failed, perhaps the USSR would manage to take Istanbul. It would take some very low probability events, but I can see the USSR taking it.

Alternatively, in the event of a third world war--non-nuclear--perhaps Turkey could fall to the USSR. So--latet date is VERY late...
 
Title is pretty much self-explanatory. How late could it have been plausible for a non-Christian, non-Muslim power to take Constantinople and hold it for a while afterwards?
(If you don't think it's possible at all, I suppose you can always answer 622 CE...)

What about say, Constantinople falling to an atheist state a la the USSR? In that case, Constantinople could fall as late as 2016. All we need is for it to stay called Constantinople. If a Byzantine Empire is required that's fine. Have the Byzantine Empire trudge on until the modern day and then get knocked out of the Great War by a joint naval invasion from the the Socialist Republic of France and the Danubian League State. (Insert weird alternate nations here)

Otherwise Mongols are probably the best bet.

P.S. In White Huns timeline, Constantinople is continually falling to various non-Christian powers. It's pretty much a staple of the plot. In fact, I plan to end the whole timeline with the fall of Constantinople in 2016 by [REDACTED].
 
1243 to Molgols

That sounds like the best option if we're talking about non-Muslim and non-Christian powers.

Or maybe the Cumans (who, IIRC, were still very much pagan) could exploit the weakness and instability of the Latin Empire and stage their own coup/invasion of Constantinople around that time.
 
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