All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
"Side with your brother, oppressor be he or oppressed" is an expression taken from a sound narration reported by Al-Bukhari . The whole narration is a follows: Anas narrated that the Prophet said: "Side with your brother be he oppressor or oppressed". A man said: 'O Prophet of Allaah! I side with him if oppressed. What if he is the oppressor; how could I back him?' The Prophet replied: "Prevent him or hinder him from oppression. Indeed that is backing him." As for the verse you mentioned it is from the chapter Al-Anfaal. Allaah says (interpretation of meaning): {And let not those who disbelieve think that they can outstrip (escape from the punishment). Verily, they will never be able to save themselves (from Allaah's punishment). And make ready against them all you can of power, including steeds of war (tanks, planes, missiles, artillery) to threaten the enemy of Allaah and your enemy, and others besides whom, you may not know but whom Allaah does know. And whatever you shall spend in the Cause of Allaah shall be repaid unto you, and you shall not be treated unjusty.}[8:59-60]. In this verse Allaah orders His believing slaves to prepare, utilizing all means of power and strength in all fields of life, including the material and spiritual strength. The most important strength is that of faith and Islamic creed, then the strength of unity and cooperation, then the military and economic powers. This strength should have no limits. It should represent a force of dissuasion that terrorifies the enemies of Islam wherever they are. This matter is not supererogatory. It is an obligation upon Muslims to yearn for such force otherwise they all commit a sin the severity of which differs from one person to the other.
Allaah knows best.