An established Islamic force is allowed to fight only those enemies who are engaged in fighting it, without committing any sort of aggression!

Verses 190-193 of Chapter 2:

And fight in the way of Allah with those who fight against you but do not commit aggression because Allah does not like aggressors. Fight against them wherever they confront you in combat and drive them out from where they drove you out. Though killing is bad. persecution is worse than killing. Do not fight against them near the Masjid Haram unless they attack you there. And if they attack you first (even in that sacred area), strike them (without any hesitation); this is the due punishment for such disbelievers. If, however, they desist from fighting (you should also do likewise), and know that Allah is Forgiving and Merciful. Go on fighting with them till there is no more a state of tribulation and Allah's way is established instead. Then if they desist from it, there should be no more hostility except against those who had been guilty of cruelty and brutality.(you should also do likewise), and know that Allah is Forgiving and Merciful.

Comments: God allowed the believers to fight under the above conditions only in Medina where an Islamic state was established under the leadership of the last prophet, but not in Mecca where the disbelievers had prevented them from openly spreading the message of God and started torturing them before they had to migrate to Medina.