All the modern and historical militaries I know about utilize some sort of top-down command structure, like an upside-down tree. Have other forms of organization been tried, and were they at all successful?
I am curious to see an answer from a military historian here, I did study myself a lot of military history (ex Army Warrant Officer here) and can't recall any examples. No matter how simple I always came across some sort of hierarchy even if as basic as 1 guy leading a tribe but that is still a hierarchy. Simplest one I can recollect may be the vikings and some Germanic tribes but they still have a chief and some prominent figures, at least internally, until in combat all broke loose.
A more recent example could be in the Spanish Civil war were large units of Anarchists organized in Columns that had up to 2000-5000 militia members, so a kind of Brigade. You would think that Anarchist were far less structured by definition but the truth is that the still answered to Army Central Staff and had their officers etc.