My understanding is that there was a historical process, or more likely a collection of historical processes, from the Greco-Roman world where unwatered wine had an ABV of around 4% and was usually only consumed watered (Why did Greeks and Romans dilute their wine?), to wine increasing strength to its prior standard strength of 12%, to liquors at 40%, to much stronger liquors, up until absinthe was near absolute alcohol with added ingredients.
Assuming the above is correct (or correcting anything in it that was wrong), what were the maximum alcohol levels over time that were available in wine? How did distillation to 40% (or more) develop?
--EXPANSION / CLARIFICATION--
In years past I've read Sally Fanlon's Nourinhing Traditions, and one of the things it discusses is "lacto-fermentation" versus "yeast fermentation." The difference is present today in that sourdough breads are lacto-fermented while most other breads are yeast fermented.
I have tried making some of the water kefirs and other drinks outlined in the book, and my understanding is that a water kefir fermented with more sugar than it needs will produce a drink that is about 2% alcohol, and is more filling; it's much easier to stop at one five ounce glass than with modern commercial use. When I visited France, I noticed that there was both "doux" (mild, 2%) and "brut" (strong, 5%) cider, and I've missed being able to get a bottle of doux cider in liquor stores here even at stores boasting a huge selection of beers and ciders. The text, among many literary references, quotes Shakespeare referring to "small beer" as a humble creature, and talks about how someone privileged talked about small beer as appropriate to their social class, and leaving "strong beer" to the miserable poor, "strong beer" being the kind of drink we call beer today. The impression I get under the book, which admittedly grinds an axe against stronger alcoholic beverages that I cannot account for on the author's stated premises, is that lacto-fermentation is common and beverages that topped out at 1-2% ABV undiluted were a considerable fixture.
(The recent popularity of kombucha reflects a fermentation that does not reach 5% or 12% ABV if fermented with more sugar than will be consumed by the culture, although I do not know the relation between kombucha and classic lacto-fermented beverages; guesswork might suggest that it is a re-introduction in modern culture of a traditional lacto-fermented beverage, but that is only a guess.)
To the best of my knowledge the comments here and the Wikipedia article on wine do not reflect a familiarity with lacto-fermented beverages with a substantially different drinking experience (one is less tempted to drink and drink and drink lacto-fermented beverages while with modern alcoholic beverages you more enforce a limit that does not impose itself on your consumption of alcohol). I remember in France hearing at a Champagne museum historical references to the process of raising the alcohol content in wine; I do not recall numbers quoted, but it is consistent with my knowledge of lacto-fermented beverages that 4% is a believable ABV for wine lacto-fermented where the culture is allowed to have its run. And I am under the strong impression based on Nourishing Tradition's that there were a number of traditionally used fermented beverages that were at 1-2% alcohol, and that for some a bottle of the lacto-fermented bottle was used like Gatorade and its competitors today.