Because aggression is not condoned, only retaliation, righting back.
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YUSUFALI: Except those who join a group between whom and you there is a treaty (of peace), or those who approach you with hearts restraining them from fighting you as well as fighting their own people. If Allah had pleased, He could have given them power over you, and they would have fought you: Therefore if they withdraw from you but fight you not, and (instead) send you (Guarantees of) peace, then Allah Hath opened no way for you (to war against them).
PICKTHAL: Except those who seek refuge with a people between whom and you there is a covenant, or (those who) come unto you because their hearts forbid them to make war on you or make war on their own folk. Had Allah willed He could have given them power over you so that assuredly they would have fought you. So, if they hold aloof from you and wage not war against you and offer you peace, Allah alloweth you no way against them.
SHAKIR: Except those who reach a people between whom and you there is an alliance, or who come to you, their hearts shrinking from fighting you or fighting their own people; and if Allah had pleased, He would have given them power over you, so that they should have certainly fought you; therefore if they withdraw from you and do not fight you and offer you peace, then Allah has not given you a way against them.
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YUSUFALI: And fight them on until there is no more Tumult or oppression, and there prevail justice and faith in Allah; but if they cease, Let there be no hostility except to those who practise oppression.
PICKTHAL: And fight them until persecution is no more, and religion is for Allah. But if they desist, then let there be no hostility except against wrong-doers.
SHAKIR: And fight with them until there is no persecution, and religion should be only for Allah, but if they desist, then there should be no hostility except against the oppressors.
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YUSUFALI: Fight in the cause of Allah those who fight you, but do not transgress limits; for Allah loveth not transgressors.
PICKTHAL: Fight in the way of Allah against those who fight against you, but begin not hostilities. Lo! Allah loveth not aggressors.
SHAKIR: And fight in the way of Allah with those who fight with you, and do not exceed the limits, surely Allah does not love those who exceed the limits.