As I recall learning many years ago from a little game theory, when you go up against unprincipled opposition, your best course is supposedly to absorb the first 'cheat'. But after that if they don't return to the rules you take the gloves off and beat them anyway you can because it's always a losing proposition to offer principled treatment to those who won't observe principle. (which in the end is why the way the Allies finished WWII was the correct course, but that's another story....).
From larger standpoint of Christian ethics this is not a trivial dilemma to face. We are taught to give our cloak to the man that takes our coat. The traditional justification out of the passive political stance is that the responsibility of an individual is different than the responsibility of civil leadership, because justice is a requirement of moral civil leadership, and if course means must be employed to remove unjust leadership, so be it. That's the theory anyway.