Taunts faints and interception.
I was having this debate with one of my friends just last month. I just couldn't agree with his point of view that it's okay for a nage to perform a preemptive strike against an aggressor. Although, there are ways to get an aggressor to attack you therefore providing you the moral, and legal, justification for counterattacking.
If I'm in a situation where I cannot reach a resolution through peaceful means, and I feel an attack is immanent, I usually try to provoke the aggressor to commit himself on my time. I don't want to continue arguing until an opportunity presents itself so that the other person can sucker punch me. We've all seen it happen, two people are arguing in a club, one of the guys diverts his attention for a moment to talk with a friend and the other person uses that as a opportunity to sucker punch the guy.
I want to aggressor to attack me on when I'm ready. I'll subtle switch into a stance, this limits the aggressors attack options, and I keep my hands up by pretending to gesture with my hands as I'm talking (A good thing to do in any confrontation). I then begin to taunt the guy, by talking about his lack of manhood or his mother. This usually works because most people are very sensitive and hyper-emotional about these topics. This usually also causes the guy to over extend himself.
During the fight, I usually use faints to draw out a response from the uke or better yet I motor-step the guy. I get him locked into a rhythm of easy to defend strikes and then break the pattern in the middle by doing a technique.
Bruce Lee in the Tao of Jeet Kune Do (The art of intercepting the fist) talks about a level of mastery in which you can sense the enemy's attack before he launches a strike. He proceeds to talk about intercepting the aggressor's attack and launching preemptive strikes. The difference between this and just attacking the guy because you feel threaten is subtle but important. In the first you sense the strike a fraction of a second before it is launched. It is almost synchronistic, acasual. The latter your fears get the better of you and you attack the guy because you feel threaten.
In all these instances you have not attacked but you have taken the initiative to elicit a response. I don't knwo whether it's aikido, but it's better than just attacking someone.
On a similar note, when has the fight begun? When the guy has stated his intention to strike you, when he proceeds to violate your space or when he actually strikes you?