Flank Contact: Contacts that are not allowed.

Well I like it (but then I would say that wouldn’t I).

What do others think?

Yes, there is nothing unreasonable or undesirable about the Roman cavalry wanting to attack the closest light horse.
But are we not forgetting what a ‘threat zone’ actually is?

Almost all table-top wargames use the turn based system, where one army moves while the other army is stationary.
Now some wargames, such as ‘Impetus’, allows the stationary army to react or counter charge during the moving army’s turn. This is to simulate the fact that in reality troops would be moving simultaneously, and would not just sit there passively as an enemy manoeuvred about in front of them.

DBA likes to keep things simple and does not have a ‘counter charge’ mechanism, but uses ‘threat zones’ to get roughly the same effect. They represent the region just in front of an element that is facing a potential threat, a danger, one that can’t be ignored.

So three points:-
1) Should the Roman cavalry (in this particular situation) be allowed to suddenly ‘forget’ about that potential threat from enemy element LH Z, the very threat that was limiting the Roman cavalry’s options before they started moving and entered the threat zone of LH Y?
2) Would the Numidian LH Z element just sit there and do nothing as the Romans freely manoeuvred about in front of them, exposing their vulnerable flank to attack? (The Romans know this is a danger, otherwise element LH Z’s ‘threat zone’ wouldn’t have been much of a threat in the first place).
3) Could the Roman cavalry commander, having committed his men to charge enemy LH Z, the one that has them pinned in a threat zone, be able to suddenly countermand that order and swing the entire body in a different direction at a moments notice without becoming hopelessly disordered when they enter the new threat zone generated by enemy element LH Y?

Personally, I think the answer to all three of these questions in NO.

So having elements or groups only responding to TZs in which they start or to the first they enter gives ‘threat zones’ their proper function on our wargames table.

I'd be wary of making such an assumption.
Stevie is an astute guy, who has been playing DBA for a very long time.

Martin

Remind me, what year was Waterloo?

Unless you are averse to sarcasm, I don't think I was making a personal insult, I am trying to: 1) point out the weakness of the argument by offering an alternative; and 2) criticize the relevance of your method - should/imagination. 

Sans-sarcasm... The point is that "should" has nothing to do with it, nor does your imagination in regards to how history might have been.

These are published rules - rules that are especially good for clubs and tournaments where a common understanding of the rules has high utility.

As demonstrated by the 2.0, 2.2, etc. transitions, when confronted with an ambiguity, sometimes we can infer the authors intention, sometimes we can deduce from the other rules.
But, if that doesn't work, we need a ruling. That might come from the author, but a unique problem with DBA is that the author has often chosen not to engage with/understand the question. So players have resorted to official channels like the 3.0 FAQ, or the unofficial like the WADBAG 2.2 guide.

Our imaginations are neither compelling or relevant.

(I'm not against a well sourced argument form history. I love that stuff. It would be a useful input into a discussion about the realism of the rules, but its off topic.
For Roman and Numidian cavalry interactions I suggest Sallust's Jurgurthine War, but I've never seen a Latin rendering of "threat zone" so I doubt we'll find much.)

This question has yet to be answered.
I think that the Cav is destroyed myself.

I too think that Cav A is destroyed due to an enemy in front edge contact with its flank.

Re the knottier question of responding to multiple TZ's:

The DBA 3.0 project strove to create rules that modeled rear world realities but in a playable manner. These considerations sometime worked together but in many cases needed to be balanced to produce a workable game.

As to real world considerations, I think on balance they favor the single response model. The difficulties of a commander having already ordered troops to respond to a near by threat then recognizing a new threat, deciding which presents the greater threat, forming a new plan of action and then relaying this new plan of action to the troops in time for them to stop doing what they were doing and start doing something else seem overwhelming. Its the old saw: order, counter-order: disorder. Examples of "voice command" period troops including those at Waterloo are germaine to the discussion about the abilities of troops to alter course while in close proximty to enemies.

The practical considerasions also favor the one response model. Its already difficult to define "responding" to a TZ and even more difficult when an element can begin responding to one TZ then enter a second and alter course to "respond" to that TZ. Its a lot of potential zig zagging.

Overall I'm still leaning toward the one response model.

Thomas J Thomas