Russia is a Mongol horde. Suing for peace against tribal border raiders (cooperate) is a rational strategy. If they know you'll always cooperate, then their best strategy is to always come back and attack (defect). Each time you achieve peace you lose land.

So, rational strategies are bad. You want to adopt an irrational strategy where you appear to be a crazy person willing to hurt themselves to get revenge - now attackers won't risk themselves in the first place.

This is why societies develop an "honor culture" and why NATO has that mutual defense clause.