Pacifism may sound reasonable. Peace is and should be desirable, war is and should always be perceived as fundamentally evil.
But pacifism doesn't make sense in an imperfect, violent world. A pacifist would never retaliate against nazi Germany, for instance. Even at the risk of sounding contradictory, violence is needed, if only for protection against a violent, fortuitous enemy.
Tit for tat is an English saying meaning "equivalent retaliation". It is also a highly effective strategy in game theory for the iterated prisoner's dilemma. It was first introduced by Anatol Rapoport in Robert Axelrod's two tournaments, held around 1980. An agent using this strategy will initially cooperate, then respond in kind to an opponent's previous action. If the opponent previously was cooperative, the agent is cooperative. If not, the agent is not. This is similar to superrationality and reciprocal altruism in biology.
This strategy is dependent on four conditions that has allowed it to become the most successful strategy for the iterated prisoner's dilemma:
1. Unless provoked, the agent will always cooperate
2. If provoked, the agent will retaliate
3. The agent is quick to forgive
4. The agent must have a good chance of competing against the opponent more than once.
In the last condition, the definition of "good chance" depends on the payoff matrix of the prisoner's dilemma. The important thing is that the competition continues long enough for repeated punishment and forgiveness to generate a long-term payoff higher than the possible loss from cooperating initially.
from Wikipedia: http://en.wikipedia.org/wiki/Tit_for_tat