This means who fight you fight him , also when you quote from Qur'an you must read the whole Surah like The Cow (Al Bakara) because it's like this.(Sorry bad English on explaining)
Qur'an(2:190)"Fight in the cause of Allah those who fight you, but do not transgress limits; for Allah loveth not transgressors."
Qur'an(2:191)"And slay them wherever ye catch them, and turn them out from where they have Turned you out; for tumult and oppression are worse than slaughter; but fight them not at the Sacred Mosque, unless they (first) fight you there; but if they fight you, slay them. Such is the reward of those who suppress faith."
I hope you got it without explaining it and I can't explain all this 1 time that needs like 1 hour to see this.