Chainmail (3rd Ed., 1979) actually has two separate morale mechanics: one on p. 15 for post-melee morale (including effects like simply being pushed back a half move or so), and a second on p. 17 for total casualties throughout the battle (which results in immediate elimination). Rules for the former include clauses on what happens if a retreating unit contacts friendly troops (causing disorder or rout in the other unit), but nothing for contact on enemy units. The snippet above is from the second rule section, which says briefly that if "no route of retreat is open to it", then the unit surrenders instead of escaping -- but no guidance is given on what qualifies as an acceptable "route of retreat", or how complete a cordon is needed for a unit to be "surrounded". In these rules that passes as academic, because in either case the routed unit is immediately removed from the table when they fail that excess-casualty morale check.
Swords & Spells
Gygax's Swords & Spells rules was actually my first encounter with miniatures wargaming as a kid. Above you see the two cases in the morale rules there where a forced retreat can happen; in both situations, there is a one-line clause similar to Chainmail about what happens if the unit is "unable to retreat" (respectively: stand and fight at a penalty, or abject surrender). But what exactly constitutes being "unable to retreat"? That's still a raw judgement call. See also the note on the very last line: in both situations the unit "cannot change formation or face", which would seem to rule out any of the cases in the last post where there's contact on all 4 sides. Keeping that particular formation block, the routed unit would seem to be "stuck" by even a minimally wrapped-around enemy.
In Doug Niles' 1st version of Battlesyststem (the 1985 boxed set), he adds a little more detail to the situation than Gygax did. Here he indicates that as long as there is a gap of 1" extent -- about the width of one single figure -- than the entirety of any routing unit is allowed to escape through the bottleneck, irrespective of the size of the unit. This is actually how I myself interpreted Gygax's rule in Swords & Spells when I initially played with that ruleset. However, at some point it became deeply dissatisfying. If a relatively large unit routs, it seems nonsensical for all the figures to be able to file out through a tiny gap without the enemy being able to stop the action. Game-wise, if you added up the time for all the movements of the separate figures, it couldn't possibly happen within one turn; realism-wise, it fails to simulate "crowd crush" and panic-type situations in those sorts of bottlenecks. Also, since only a completely-surrounded unit could be stopped from escaping, it made capturing an enemy unit effectively impossible (the ability to get figures on every outer inch of a unit, without attacking and routing the target too early, is basically nil).
Here is Doug Niles' second take on his mass war rules for D&D (1989 perfect-bound glossy book). Seemingly sharing my dissatisfaction with the previous naive interpretation of Gygax, he's a little bit more strict here. On the one hand, he says "a routed unit cannot change frontage [as in Swords & Spells], unless that is the only way it can perform its rout movement" -- but that's just a different way of saying "a routed unit can change its frontage [the opposite of Swords & Spells]". But instead of the previous one-figure gap requirement, here you need a gap equal to "the widest stand of figures in the unit". In these rules, "stand" means a combined base of either two figures for cavalry, or three for infantry. So now that's probably about 2" or 3" wide and then a whole unit can escape. So it's a difference, but not a big one.
I should say that, looking at the preceding, there's a lot of continuity in the mass-warfare rules published by TSR. We could perform an exercises similar to "Spells Through the Ages" and track specific rules or even blocks of text that are basically copy-and-pasted forward from Chainmail, to Swords & Spells, to Battlesystem, with certain edits or expansions along the way (much like this "when do you get blocked from retreating" rule). For some contrast, let's look at the other gorilla in fantasy wargaming:
Warhammer 6th Ed.
A lot of the sensibility is the same here -- a badly-damaged unit is forced to retreat, and is in some danger of being totally destroyed ("if caught by pursuers", i.e., the overrun rule, last paragraph above). Much like Chainmail, there is a treatment of running into friendly units on the move (moving around or even through, 4th paragraph), but not of running into enemy units. Also much like Swords & Spells, the text gently suggests that for fleeing units you should "keep them in formation" (2nd paragraph), which again if enforced strictly would prevent the retreat of a unit with even one or two enemy figures on its backside.
I can't find any more detail on the rights or restrictions for retreating units than that in these 6E Warhammer rules. Again we seem mostly in a judgement-call situation about how small a gap is necessary for a large mob of creatures to escape.
ConclusionsPersonally, I'm pretty surprised that I can't find any more detail to this situation in any of these miniature wargame rules than that. No wonder I've been frustrated for so long. The naive interpretation of Gygax's short comment in Swords & Spells, as implemented in Battlesystem, seems unsatisfyingly generous, and doesn't seem to resemble real "crowd crunch" situations.
What do you think the best expression of the allowed-retreat rule should be? Do any of these rulebooks synch up with your intuitions from the prior blog post?