Allah hath purchased of the believers their persons and their goods; for theirs (in return) is the garden (of Paradise): they fight in His cause, and slay and are slain: a promise binding on Him in truth, through the Law, the Gospel, and the Qur’an: and who is more faithful to his covenant than Allah? then rejoice in the bargain which ye have concluded: that is the achievement supreme.
 It has been a principle enshrined in all divinely revealed scriptures that the righteous have to fight against the wicked. If they don’t, then wickedness spreads, and innocent men, women and children suffer under the yoke of evil and oppression. The Torah is full of instances where the righteous were exhorted to fight against evil, while the New Testament echoes this theme in Hebrews 11:32-34 and also when Jesus famously stated, “Do not think that I came to bring peace on the earth; I did not come to bring peace, but a sword. For I came to set a man against his father, and a daughter against her mother, and a daughter-in-law against her mother-in-law; and a man’s enemies will be the members of his household.” (Matthew 10:34-36) The caveat, however, that sets apart war for God’s sake and distinguishes it from mere war for conquest or loot is that a just war in the name of God is to stop oppression, so that all people may live in peace and security with regard to their lives and property. All other causes for war are fueled by greed, and men fighting for such causes know no limits and conduct themselves like barbarians. The war for God’s sake envisions peace at the end for both the conquered and the conquerer, while war for greed envisions nothing but the destruction and subjugation of one’s enemies. Sometimes people falsely label their war for greed as a war for God, such as in the Christian Crusades or the campaigns of the nominally Muslim warlord Timur the Lame. In that case, even such “holy warriors” must be opposed because they fight for the things of this life. The Prophet was once asked who fought for God: the one who fights for fame, fortune or adventure, and the Prophet replied that the only one who is counted as fighting for God is the one who fights only to uphold His religion and for His sake alone. (Bukhari, Muslim)
Emerick, Yahiya. The Meaning of the Holy Qur’an in Today’s English (p. 829). Unknown. Kindle Edition.