Allowing my 5 yr old to hit back................

This use to be an issue between Andy and I. I was all about talking your way out of everything or simply just walking away. I didn't want my sweet little boy to hit at all. Andy quickly explained to me that if we don't teach him to be comfortable with standing up for himself now, he won't ever do it. I believe in resolving things early before it becomes a habit so I understood and agreed. We have always taught A.J. not to hit girls or small children but there are some girls and small children who try to bully him that are going to learn a lesson. I know it isn't our place to teach someone's kids a lesson. But it is our job as parents to make sure they can defend themselves and give them the tools to do it. A couple years ago, A.J. was playing with a friend's toy. The friend is a little over a year younger than A.J. but very big. She/He came to take her/his toy back and A.J. wasn't done. The friend began pushing and A.J. said stop and put his hand on the friend's chest to hold her/him away. The friend began screaming and hitting and A.J. hit her/him back. I felt bad for a second and realized A.J. had to show his position on the matter. Sometimes I'm afraid people view him as a bully because he is older but he is still just standing up for himself. If A.J. were to hit an older kid I hope they would hit him back so he would know what their position is. Trust me, my kid doesn't just go around hitting for no reason. He has to be bullied or taunted. We are going to put A.J. in some kind of karate class so he can learn when it is acceptable to fight back, and how to do it. I would like to hear some opinions on this matter.