Proposition 4. For a nice strategy to be collectively stable, it must be provoked by the very first defection of the other player. - Robert Axelrod

The live-and-let-live system that emerged in the bitter trench warfare of World War I demonstrates that friendship is hardly necessary for cooperation based upon reciprocity to get started. Under suitable circumstances, cooperation can develop even between antagonists.

Just as important as getting cooperation started were the conditions that allowed it to be sustainable. The strategies that could sustain mutual cooperation were the ones which were provocable.

The cooperative exchanges of mutual restraint actually changed the nature of the interaction. They tended to make the two sides care about each other's welfare. This change can be interpreted in terms of the Prisoner's Dilemma by saying' that the very experience of sustained mutual cooperation altered the payoffs of the players, making mutual cooperation even more valued than it was before.