Playground Politics

Today my little one was at the playground staring into this hypnotic circle thing that spins round and round by someone who turns it on the other side.  A little girl came by and wanted to stand right in front of it and put her hands on it.  So of course my son wanted to do the same.  But I could tell she did not want to share the space.  I don't know exactly what she said because I wasn't close enough to hear but I could see she was trying to block him and get him to go away.  But he's pretty persistent.  She then started to physically push him away.  And he went right back to stand next to her.  It was weird because she let this other little girl stand next to her and touch the circles but she was totally anti my son doing the same.  When she pushed him hard enough so that he fell to the ground, I thought that was a bit far so I pulled him up to his feet to take him away and I told the little girl that that wasn't very nice and that she shouldn't be shoving other people.  No parent/caretaker seemed to be around for her.  But I tried to drag my son away and he got upset and started crying.  There were other caretakers there who said I shouldn't move him away but speak to the girl to make her understand she should share the space and let others play there.  I just wanted to avoid conflict with a 3 year old or however old she was.  Am I gearing him up to get bullied in the future because I didn't try to make him stand his ground?  I just felt so awful he got pushed and I felt like he didn't understand why he got shoved to the ground.   Eventually he did stand there again.  The bully little girl went somewhere else to play and eventually he lost interest and we decided to go home.  But it really did just break my heart.  I don't want my kid bullied when he gets older and is alone at school or whatever.  I won't be there to watch over him.  I don't want to have to worry about that.  I also don't want him to be the bully because might that happen if he gets shoved around enough?