For me a game mechanic that reflects the unpredictability of battle, e.g. troops running out ammo, is great (I've nothing against well balanced random events).
My problem with this card, is precisely that it is NOT a random event and does not feel like one.
I had just about come to terms with it as gamey but acceptable in the original card deck, because at least playing it meant that you forfeited an alternative, which in itself could be punishing.
But placed in the tactics deck, it's impact is greatly and, IMHO, unnecessarily multiplied.
I played an epic version of Talavera last summer and I ended up playing Short Supply to break up my opponent's cavalry attack (lancers 'teleported' to the rear), whilst simultaneously unsquaring my infantry (can't see anything in the rules to stop me doing that) and successfully counterattacking.
Now as a random event, I could handle that, but as a preplanned move, it felt cheap and underhand. I eventually won the battle by a small margin, but felt like my victory had been sullied by playing that card. My opponent was extremely chivalrous about it, but I felt embarrassed as the more experienced player. Of course, we could come up with a narrative to explain this misfortune for the French, but the reality is that I should not have been able to combine the planning of my counterattack with an event that I could not have foreseen.
Now of course I can accept that it is a game and the playing of cards is an aspect of the game play and I respect the fact that RB has found a way to introduce interesting non-standard events within the general flow of the game (that's why I play almost nothing but his games as I think they are more playable and offer more plausible battles than any other system I've tried), but, leaving aside the problem of simulation, I simply think that this card is too powerful as part of the tactic deck.
In short, my personal preference, would be to omit or change this card. My suggestions are:
1) Agree with your opponent(s) to put this card aside.
2) Agree with your opponent(s) to only play this card as a default tactic card, i.e. move a leader.
3) Agree to play this card with an alternative effect - one suggestion as an alternative to the one previously proposed would be to play this card when your opponent declares a ranged attack: opponent battles normally, but immediately afterwards the battling unit is moved to a rear hex. That for me would keep the feel of the card, while reducing its power and giving it a more obvious relationship to battlefield events. I do also like the idea of it being played as a 'random event' card as FrenchricusRex suggests, but still feel a little uncomfortable about completely free choice about which unit to move, especially as you might well pick it up directly before your own turn. I definitely don't like the idea of being able to move your opponent's units completely freely (I would stick to the teleporting to the baseline or 4 hexes towards the baseline).
My absolute preference would be for RB/GMT to issue an alternative rule/replacement card that everyone could agree on...
If everyone made war only according to his own convictions, there would be no war.