Cohesion was understood to be of great importance for combat units' abilities long before Martin van Creveld added it to the more academic debate with his book "Fighting Power" in 1982.
The many different remarks, observations and descriptions of cohesion and its role from different sources didn't fit together in my mind, though. Something was missing, it was more an ill-defined cloud than something well-understood.
Well, yesterday I remembered how Franz Uhle-Wettler in his (recently re-read) book "Gefechtsfeld Mitteleuropa" favoured the USMC rifle squad of 13. He had remarked (also about 30 years ago) how the fire teams in this U.S. style squad were the embodiment of the much-praised "kleine Kampfgemeinschaft" (~small combat companionship), after all. "Kleine Kampfgemeinschaft" is a German keyword for the pursuit of small unit cohesion.
Not sure why, but this was the final trigger for me to understand the whole thing (or to believe I do), and I want to share it:
There are -according to my impressions from literature and practice - three different faces of cohesion. They exist on different levels and serve different purposes. "Cohesion" as an over-arching buzzword is being used for all three, in many sources only for one or two. This explains why it was such a foggy thing to me.
The low-level cohesion is really the "kleine Kampfgemeinschaft". This applies to buddy team (2), fire team (4) and squad (6-13) levels.
This low-level cohesion helps against the stress stemming from hardships. Hardships of field work, environment, sleep deprivation, the dangers of combat and the horrors of combat and its aftermath.
The middle-level cohesion applies to platoon and company levels.
This kind of cohesion is for social cohesion. The troops eat together, have common low-level administration, the same company sergeant ("Spieß") - this forms a small community.
The best example for this kin of cohesion that I know is the observation that lightly wounded or sick soldier preferred to stay with their company whenever possible, trying to avoid the social isolation in a field hospital.
Finally, there's the high-level cohesion. This applies to the range from battalion up to corps. It's often being built by having military recruitment districts. The old German army system knew "Wehrkreise", each of which raised a corps. The Italian mountain troops (Alpini) regiments recruited their personnel from their very own Northern Italian recruitment area until a couple years ago.
The utility of this kind of friction is very different; it's about reducing friction and reducing the tendency towards harmful egoism.
Common nationality (ethnicity), language, training, methods, terminology and (regional) culture are its drivers.
I hope this will help readers to navigate through whatever they'll read about cohesion in the future, allowing for more clarity and utility.