I might of been too convoluted in my Original post.
Original post edited:
1) Allowing the Defender to pull his surviving forces back, giving up the contested territ.
2) Allowing the attacker to withdraw from Combat before losing all his units while using the Auto assault button.
Defending Army has the option to withdraw once his force size reaches a designated (by controlling player) number.
Attacking Army, having the option to withdraw (stop assaulting) from Combat once his army size reaches designated number.
Allowing players to drop a number onto the Auto assault button; this number would represent the force reduction an individual army has to lose before that army can withdraw from battle.
Defending Army would automatically retreat to adjacent terit that has the largest amount of friendly units, ties would be sent to random adjacent friendly controlled terit.
Allowing (or not) the assault (Non-auto assault) button to be used in preventing defender withdrawing from combat.
This might give the game of Risk a different feel, possibly making it feel like a real War game?
Regardless of the defender withdrawing from battle, I think you should give the attacker the option to withdraw from combat.
P.S. Optional: Defending Army will retreat before battle ends, thereby leaving one unit behind, allowing Attacker the choice; attack for control or leave control to the defender.