War is only justified in self-defense and to fight oppression.
Rules of War
2:190 You may fight in the cause of GOD against those who attack you, but do not aggress. GOD does not love the aggressors.
2:191 You may kill those who wage war against you, and you may evict them whence they evicted you. Oppression is worse than murder. Do not fight them at the Sacred Masjid, unless they attack you therein. If they attack you, you may kill them. This is the just retribution for those disbelievers.
2:192 If they refrain, then GOD is Forgiver, Most Merciful.
2:193 You may also fight them to eliminate oppression, and to worship GOD freely. If they refrain, you shall not aggress; aggression is permitted only against the aggressors.
2:194 During the Sacred Months, aggression may be met by an equivalent response. If they attack you, you may retaliate by inflicting an equitable retribution. You shall observe GOD and know that GOD is with the righteous.
2:195 You shall spend in the cause of GOD; do not throw yourselves with your own hands into destruction. You shall be charitable; GOD loves the charitable.
If possible the first thing to do before going to war is to get up and leave if you can.
4:97 Those whose lives are terminated by the angels, while in a state of wronging their souls, the angels will ask them, “What was the matter with you?” They will say, “We were oppressed on earth.” They will say, “Was GOD’s earth not spacious enough for you to emigrate therein?” For these, the final abode is Hell, and a miserable destiny.
As a submitting nation prior to going to war with another nation a better approach would be to open immigration for people who live in oppressive societies to be able to leave. This can create an exit for people who value their faith more than the land.
59:9 As for those who provided them with a home and a refuge, and were believers before them, they love those who immigrated to them, and find no hesitation in their hearts in helping them. In fact, they readily give them priority over themselves, even when they themselves need what they give away. Indeed, those who overcome their natural stinginess are the successful ones.
17:26 You shall give the due alms to the relatives, the needy, the poor, and the traveling alien, but do not be excessive, extravagant.
9:60 Charities shall go to the poor, the needy, the workers who collect them, the new converts, to free the slaves, to those burdened by sudden expenses, in the cause of GOD, and to the traveling alien. Such is GOD’s commandment. GOD is Omniscient, Most Wise.
If it is not possible to exit an oppressive country and the rulers of that country will not allow the people who want to leave to leave then war can be justified.
T he Believers are Fearless
4:75 Why should you not fight in the cause of GOD when weak men, women, and children are imploring: “Our Lord, deliver us from this community whose people are oppressive, and be You our Lord and Master.”
4:76 Those who believe are fighting for the cause of GOD, while those who disbelieve are fighting for the cause of tyranny. Therefore, you shall fight the devil’s allies; the devil’s power is nil.
4:77 Have you noted those who were told, “You do not have to fight; all you need to do is observe the Contact Prayers (Salat) and give the obligatory charity (Zakat),” then, when fighting was decreed for them, they feared the people as much as they feared GOD, or even more? They said, “Our Lord, why did You force this fighting on us? If only You respite us for awhile!” Say, “The materials of this world are nil, while the Hereafter is far better for the righteous, and you never suffer the slightest injustice.”
9:38 O you who believe, when you are told, “Mobilize in the cause of GOD,” why do you become heavily attached to the ground? Have you chosen this worldly life in place of the Hereafter? The materials of this world, compared to the Hereafter, are nil.
Usury is a form of oppression
2:278 O you who believe, you shall observe GOD and refrain from all kinds of usury, if you are believers.
2:279 If you do not, then expect a war from GOD and His messenger. But if you repent, you may keep your capitals, without inflicting injustice, or incurring injustice.
2:280 If the debtor is unable to pay, wait for a better time. If you give up the loan as a charity, it would be better for you, if you only knew.
How do we define war? Some people say that today we fight by means of the pen. Is this a form of war?
It may not be right to call the war of a pen in the same category as the people who were required to give their life to physically fight in a real war.
Which months is war not allowed for war?
God’s System: Twelve Months Per Year*
9:36 The count of months, as far as GOD is concerned, is twelve.* This has been GOD’s law, since the day He created the heavens and the earth. Four of them are sacred. This is the perfect religion; you shall not wrong your souls (by fighting) during the Sacred Months. However, you may declare all-out war against the idol worshipers (even during the Sacred Months), when they declare all-out war against you, and know that GOD is on the side of the righteous.
Do we need a prophet or messenger to have a war?
Not necessarily as long as the justification for the war meets all the Quranic requirements.