This is the quote that Civilization VI attributes to the Stirrup technology. What's so great about it? As an invention, it sounds like a no-brainer that should have been invented five minutes after inventing horse riding. "Hey, that horse is tall and hard to mount, maybe if I had something to step on that would help". And what influence did it have on history? In the tech tree, it leads to Banking and Gunpowder, none of which makes sense to me at this point. What's the thing with the stirrup?
White's stirrup thesis is, in short, a gross misunderstanding of history and has been criticized since more or less the moment it was published by other historians. Which, of course, hasn't stopped the stirrup thesis from popularity in games and pop culture. In fact, the suggestion came under such fire that the entire debate has been called "the great stirrup controversy." I'm going to briefly go over the salient points of the debate - such as it is - and talk a little more about the larger problems with technological determinism in history as they tend to be presented in many video games.
White first suggested that the stirrup was a critical piece of technological change in his 1962 book, Medieval Technology and Social Change. Since the focus of the book is on how technology led to social changes, it's probably no surprise that one of the more visible social changes White focuses on have to do with the seemingly sudden appearance of armored, mounted knights, and especially their impact on warfare. This approach, though, hyperfocusing on emergent technologies and arguing that a kind of gadgetry is at the root of enormous social and cultural behaviors, is what historians call "technological determinism." In other words, technology and invention is what drives change in cultures. While there's certainly some truth to the idea that invention can drive social change, most historians have tended to abandon the idea that technology is, itself, any kind of driver of progress. Many historians nowadays would even look pretty critically at the entire idea of "progress" in any form outside of iterative development of particular industries or technologies - historians can quite comfortably say, for instance, that modern lightbulbs are brighter and more efficient than light bulbs of say, 1920, but would balk at the implication that brighter lightbulbs are responsible for any other historical trends that might emerge between then and now.
Instead, historians tend to look at how the cultures, economic circumstances, or social trends themselves drove patterns of behavior, and if historians are still focused on single pieces of technology, then they study that technology as an element of cultural production or adoption, rather than, as White had, the inverse. All of this is to say that the idea that any one, single invention was capable of producing massive social change without some parallel changes in broader ideas at the cultural level, is pretty dubious.
White's thesis, then, should be understood as an expression, itself, of certain trends in historiography of the mid 20th century, which themselves were already starting to fade away. The primacy of the Whig theory of history championed (largely) by Victorian historians - the idea that history was a series of incremental changes that ultimately showed social, cultural, and technological progress - was coming under severe criticism even by the early 60s (at least by professional historians; the idea obviously still holds a lot of pop-cultural power). The point of bringing this up is that while we can't (and shouldn't) entirely discount the impact of technology on culture, the idea that technology or invention is somehow central to the historical story of western civilization is a very particular idea. In other words, it is biased toward a worldview that tends to see western European culture and "enlightened" market capitalism as a sort of apogee of progress, and so histories that look at how that particular society arose to dominate the world tend to hyperfocus on emergent technologies, because in western Europe and the United States in the 1960s saw technological sophistication as a particular cultural triumph.
All that out of the way, let's look at what White was arguing. The first two sentences in White's first chapter - entitled "Stirrup, Mounted Shock Combat, Feudalism, and Chivalry" - say:
The history of the use of the horse in battle is divided into three periods: first, that of the charioteer; second, that of the mounted warrior who clings to his steed by pressure of the knees; and third, that of the rider equipped with stirrups. The horse has always given its master an advantage over the footman in battle, and each improvement in its military use has been related to far-reaching social and cultural changes.
So White spells out plainly that not only do changes in emergent patterns of horse use on battlefields represent "improvements" in an objective sense, but also that those improvements are at the root of social and cultural changes. The title of the chapter is itself an argument: stirrups led to shock combat, to "feudalism," and to chivalry. White then goes on to represent a comical view of stirrupless cavalry falling to the ground like a Jenga tower after missed cuts before making the central argument of the chapter:
The stirrup make possible... a vastly more effective mode of attack.
That's the central element, and the one that is most often extruded into the popular culture. Wobbly, unsteady men gripping horses with their thighs breaking like a cresting wave against men who can steady themselves with stirrups. It's an effective mental image, and it's satisfying to an audience whose worldview largely uncritically accepts that idea that 1) history is the story of progress and 2) that progress can be measured by invention and technological change.
Criticisms came pretty fast. While allowing that the introduction of the stirrup to western Europe occurred around the same time that "feudalism" (which is itself a problematic term) and changes to the military structure of the Carolingian empire similarly occurred, but White did little to prove that the stirrup, more than about ten thousand other elements, was the central driver. Or, indeed, if there was any single central driver at all, instead of just a series of small incremental changes that spread in a more complex way. To be clear, here, historians today are still unsure about the exact whys and wherefores of all of this, but we can be pretty sure that the stirrup, by itself, was nothing more than another tool in the toolkit of a culture that was already trending toward mounted shock cavalry organized around vassalage and property rights.
In any case, only the very next year the first clapback was published, taking aim at technological determinism itself, as well as the central arguments of White's thesis. Since 1963 there have been countless takes on all of this, and most are pretty critical of White, and historians of a great many different methodologies have more or less uniformly rejected the idea that the stirrup was itself anything special in promoting armored mounted men. Even other technological arguments have suggested that instead of stirrups it was the canted saddle that helped men stay steady enough on horseback to use couched lance tactics, but rather than suggesting that the canted saddle was the technological progenitor of chivalry, historians interested in the specific conditions of tactical combat have wisely steered clear of making any arguments of the kind.
The stirrup controversy also plays out a sort of miniature version of an 8th or 9th century "military revolution" debate; arguments about gunpowder and artillery and the change from mercenary warfare to state-run armies themselves tended to start with technological determinism as their basis and have come under significant scrutiny ever since. That debate is also wide-ranging and has had numerous book and articles written about it.
But these debates among historians tend not to be as visible or as interesting in popular culture. You don't see "incremental social reforms that trend toward a diffusion of the monarch's power toward an armed and empowered class of men whose primary social duty was warfighting" as an option in Civ5 or Age of Empires. It's harder to gamify the somewhat chaotic emergence of particular trends within particular social constraints as a means to solve particular socio-economic problems than it is to just point at a particular piece of technology and say "that's what will take you out of the Dark Ages." Games are a massively popular medium of historical interpretation, and have an obvious power in spreading ideas. I think that's great! I spent a good deal of my own youth playing Age of Empires II, and I doubt I would have pursued the education and profession I did without having played games like it, but we should also be careful to separate what are elements of history that make for good or simplified gaming from good history. They are very seldom the same.
Lynn White's og thesis is in Medieval Technology and Social Change.
Peter Sawyer and HR Hilton wrote the 1963 criticism in "Technical Determinism: The Stirrup and the Plough."
Philippe Contamine wrote another criticism in 1980 in his book War in the Middle Ages in a chapter called “The Problem of the Stirrup.”
Another great thread here, from the mechanism/Equestrian side. From /u/PM_ME_UR_SADDLEBREDS