The rule of Law has been what has separated Judeo/Christian culture. I believe people should intervene when laws are broken. Laws, constitutions, and such are social contracts for mutual survival and the pursuit of happiness. If possible, citizens should do what they can to protect their neighbors... whether they like them or not.
Nations should protect their citizens from invaders and other attackers.
If you were out of town and your wife was attacked, you would hope that a good Samaritan would do what they could to help.
Jesus makes it easy, just do unto others as you would have them do unto you. I don't know how to improve on that simple guide.
So, if you decide to help your neighbor's wife from an attacker, when do you stop? Down the street? The next block over? The next city?