In Skyrim Esbern says Reman Cyrodill defeated the Akaviri in mysterious circumstances?

We now know the Akaviri were looking for a dragonborn, Did Reman use Th'um on their armies?

Why was Reman's victory so sudden?

They surrendered when they recognized Reman was a dragonborn. He did not actually defeat them in a war.

It would be like the crusaders laying siege on Jerusalem just to find Jesus standing on the walls asking them to surrender.

I guess we will never know what could prompt an entire race to launch an invasion in search of one human being. Who knows what they needed so bad from a dragonborn

They probably needed somebody to stop Alduin once he returned, and believed that only a dragonborn could do that. Why they believed that or how they would ever know Alduin dissapeared is beyond me, but it's the only thing that I can think of that would make sense. Especially when you consider the writing on Alduin's wall.