Care to elaborate? Is that not when the stirrup was developed?
That is more or less when the stirrup was introduced to Europe, yes, however
Please, do inform me of what historical fallacy I am engaging in when I state that the ability to easily remain on the horse would be related to the increased utility of cavalry.
I mean, it wasn't like I said:
"Byzantine Cataphracts roflpwnd the unorganized Arabs."
the introduction of the stirrup had no clear effect on states' willingness or ability to raise units of effective armored or missile cavalry. In most of Europe, no polity had the ability to raise large forces of such cavalry anyway for several centuries after the introduction of the stirrup, making it moot. The one European polity that could, the Byzantine Empire, had already begun to introduce large formations of armored cavalry well over a century before the stirrup was employed in the West (so large even that one recent historian of their use has argued that the Byzantine army of the era of Ioustinianos was
wholly mounted, which is going a bit far - but not by that much). The Sasanian army, which experienced the stirrup probably earlier than did the Byzantines, had possessed formations of massive armored horse centuries before that - the Iranian tradition of the
dehkan warrior-gentry provided much of the foundation for the Sasano-Parthian confederacy back in the third century. Simply put, there is no actual link between the introduction of the stirrup and any sort of tactical revolution in cavalry; usually historians tend to require a more formal causal link than "that's the sort of thing that totally should've happened".
The fascination with the stirrup is another facet of the pernicious impact of various sorts of technological determinism, which usually goes hand in hand with Whiggery (which is
prima facie bad). This particular circumstance is due primarily to the enthusiastic advocacy of Lynn Swann, who several decades ago claimed that the stirrup not only revolutionized European tactics (harrumph) but that it spawned feudal society
de novo (committing the grievous error of locating "feudalism" before the tenth century but that is incidental to the point).