I'm sorry, but i don't have the energy to read through 29 pages of replies. Maybe my thoughts on this subject have already been said, if so i do apologize, but anyway, here it goes:
I have a fairly new version of the traditional Risk board game, where this is incorporated, albeit in a slightly different manner:
When attacking, no matter how many troops you have, you are only allowed to use 3 at a time, and the defender can only use 2 at a time. You can attack over and over again, if you feel like it. So, an attack from, say... a territory with 10 troops against one with 6 defenders, could look something like this:
3v2, 1v2, 0v2
[new attack] (...or stop attacking)
3v2,3v0
[new attack]
3v2,3v0
[new attack]
3v2,2v1,1v1,
1v0[attacker win]
(...you're also not allowed to quit an attack 'midfight', as in until the attacking 3 or defending 2 has been eliminated)
Now, since the attacker only has one troop in
'attacking mode' after the win, this is of course the only troop thats allowed in the conquered territory (before ending attacks and going into reinforcement mode), and the 'home' territory has 4 troops left. Hm, do you get it? I'm not the best at explaining.

At the most, an attacker can have 3 troops in a conquered territory directly after a win, and if he feels like it he could of course use two of them for a continued attack beoynd this territory. But in reality this is a way of playing with adjacent attacks. And a really good one! It's very enjoyable, and it does certainly add alot of strategy when it comes to reinforcements and troop movements.
I would really like to see adjacent attacks incorporated in this way.
This is also said to equalize odds between attacker and defender, which is otherwise slightly in benefit of the attacker. I don't know the math behind this statement, it's only something that i've read.
.