Q: reasons wars were fought in history, what is a “just war”
And wars fought for the sake of peace and justice, guidance from theology, examples of unjust wars and just wars

A:

Jazakallah khair for clarifying. Let me give you a comprehensive overview directly here.

Wars in History — Causes, Justice, and Theology

WHY WARS ARE FOUGHT — ROOT CAUSES THROUGHOUT HISTORY
Human history shows wars have been driven by a recurring set of motivations:

1. Territory and Resources
   The most ancient driver — land, water, trade routes, oil, minerals. Rome’s expansion, the Mongol conquests, colonial land grabs in Africa and Asia, and even modern conflicts in the Middle East all have resource competition at their core.
2. Power and Dominance
   Empires rise by subjugating others. The Assyrian, Persian, Greek, Roman, Ottoman, British, and American imperial projects all reflect the human drive for hegemonic control.
3. Religion and Ideology
   The Crusades, the Thirty Years’ War (Catholic vs. Protestant Europe), the Arab conquests, and modern ideological wars (communism vs. capitalism during the Cold War proxy wars) show how belief systems mobilize mass violence.
4. Nationalism and Identity
   WWI was fundamentally triggered by nationalist rivalries. The Balkan wars, the breakup of Yugoslavia, and many African post-colonial conflicts stem from ethnic and national identity clashes.
5. Oppression and Resistance
   Many wars are responses to tyranny — the American Revolution, the Haitian Revolution, anti-colonial independence wars across Asia and Africa, and the Palestinian resistance all fall here.
6. Economic Exploitation
   The Opium Wars (Britain forcing opium trade on China), most colonial wars, and many modern interventions in resource-rich nations are essentially economic wars dressed in political language.
7. Pre-emptive Fear
   States sometimes go to war fearing future threat — the 2003 Iraq War being a notorious modern example of a war premised on a threat that did not exist.

WHAT IS A “JUST WAR”?
Traditional just war theory comprises two sets of requirements: jus ad bellum — the right to go to war — and jus in bello — proper conduct while engaged in war.  Some modern scholars add a third: jus post bellum — justice in building peace afterward.
The theory starts with a presumption that war is prohibited. Augustine developed it as a tool to assess the morality of wars in order to limit their number and brutality, and to protect the moral order of the world. 
The Classic Criteria for Jus Ad Bellum (Going to War):
Thomas Aquinas systematically organized the criteria, which included: Legitimate Authority — only duly recognized leaders can declare war; Just Cause — wars are just if they rectify a wrong suffered; and Right Intention — intentions must be for the pursuit of good, not self-gain. 
Beyond these three, the tradition requires:
∙ Last Resort — war must be the last resort; negotiations and compromise must have been tried and failed. 
∙ Proportionality — the harm caused must not exceed the good achieved.
∙ Reasonable Chance of Success — pointless slaughter cannot be justified.
Jus In Bello (Conduct During War):
∙ Non-combatants must be protected
∙ Prisoners must be treated humanely
∙ Weapons causing unnecessary suffering are forbidden
∙ There must be a limited objective — namely, the restoration of peace. This means the collapse of a nation’s economy and the destruction of its political institutions are unwarranted. 

THEOLOGICAL GUIDANCE
Islamic Framework — Jihad as Just War
The Islamic framework is arguably the most detailed and earliest codified system of war ethics in history.
The Quran establishes the foundational principle in Surah Al-Baqarah (2:190):
“Fight in the way of Allah those who fight you, but do not transgress. Indeed, Allah does not like transgressors.”
And Surah Al-Hajj (22:39-40) — the first Quranic permission for warfare:
“Permission to fight has been given to those who are being fought, because they were wronged.”
Key Islamic principles:
∙ Most scholars agree that difference in religion alone is never a sufficient justification for jihad.  The cause must be oppression, aggression, or persecution.
∙ The Islamic framework aims at allowing, and sometimes obligating, warfare as required to preserve the freedom and security of the Muslim community within strict legal limits to preserve human life and property, restricting violence to only what is absolutely necessary. 
∙ The Prophet ﷺ gave strict battlefield ethics: do not kill women, children, the elderly, monks in their monasteries, or farmers not bearing arms. Do not destroy trees, crops, or burn homes.
∙ Islam requires that its warriors fight only for Allah — not aggrandizement, glory, or booty. 
The four dimensions of Jihad per Ibn al-Qayyim: jihad against four types of enemies — the lower self (nafs), Satan, the unbelievers (when legitimate conditions are met), and the hypocrites.  Armed conflict is only one narrow dimension.
Christian Framework
Augustine and Aquinas built Western just war theory largely from the Christian moral tradition. Augustine believed the Kingdom of Peace cannot be realised within human history, so we must reckon with the reality of sin and violence — yet he had a deep abhorrence of war and sought to develop criteria to limit their number and brutality. 
Ancient and Cross-Cultural Roots
One of the earliest recorded instances of the just war concept appears in the ancient Indian epic the Mahabharata,  establishing criteria like proportionality (chariots cannot attack cavalry, only other chariots), just means (no poisoned arrows), just cause (no attacking out of rage), and fair treatment of captives and the wounded. 

EXAMPLES OF JUST WARS

1. The Battle of Badr (624 CE)
   The Muslims had been expelled from Makkah, their property confiscated, their people tortured and killed. This was a defensive war of survival by a persecuted community. Every condition of Islamic just war was met — legitimate authority (the Prophet ﷺ), just cause, right intention, proportionality.
2. The Liberation of Makkah (630 CE)
   The Quraysh violated the Treaty of Hudaybiyyah. The Prophet ﷺ marched — yet the conquest was executed with near-zero bloodshed and a general amnesty was declared. A model of jus post bellum — justice after victory.
3. The Allied War Against Nazi Germany (WWII)
   By any standard — theological or secular — stopping the Holocaust, Nazi expansionism, and total state terror constituted a just cause. The aggressor was clear, legitimate authority existed, and the cause was defense of the oppressed.
4. Anti-Colonial Independence Wars
   Algeria (1954–62), India’s independence movement, and many African liberation struggles were responses to brutal occupation, dispossession, and systematic oppression — meeting the just cause criterion clearly.
5. The Abyssinian King Ashama’s Protection of Muslim Refugees (615 CE)
   While not a war, King Negus of Ethiopia’s refusal to hand over Muslim refugees to Qurayshi pressure, despite political cost, is a theological model of justice overriding power — often cited in Islamic ethics.

EXAMPLES OF UNJUST WARS

1. The Crusades (1095–1291)
   While framed in religious language, the Crusades illustrate the application and challenges of Just War principles — in reality, massacres of civilians (including Jews and Eastern Christians), looting, and political power grabs violated every condition of just war. Pope Urban II’s call was exploited by feudal lords seeking land and wealth.
2. The Mongol Invasions (13th Century)
   Pure conquest, mass slaughter of civilian populations, destruction of Baghdad in 1258 (the Abbasid Caliphate’s fall), and the annihilation of entire cities like Samarkand represent war with no moral justification.
3. The Iraq War 2003
   A textbook unjust war — no legitimate international authority (no UN mandate), the stated cause (WMDs) was fabricated, it was not a last resort, and the consequences were catastrophic destabilization of an entire region. Germany’s Kriegsraison doctrine during WWII — rendering ordinary protection laws inoperative due to claimed military necessity — is a parallel example of how “necessity” arguments are weaponized to justify atrocities.
4. Colonial Wars of Conquest
   The British conquest of India, the Belgian genocide in Congo, the French in Algeria, and the Spanish in the Americas were wars of pure exploitation — no just cause, no proportionality, no protection of non-combatants. Millions died.
5. The Vietnam War
   The United States intervened in a civil conflict with no existential threat to itself, used disproportionate force (napalm, Agent Orange, carpet bombing of civilian areas), and the stated domino-theory justification proved hollow.

THE CORE PRINCIPLE ACROSS ALL TRADITIONS
Whether from the Quran, Augustine, Aquinas, or ancient Indian ethics — all serious moral traditions converge on this: war is a last resort, its purpose must be justice not gain, non-combatants must be protected, and the goal must always be a just and lasting peace — not domination.
The greatest test of a “just war” is not how it is declared — but how it is conducted and what kind of world it builds afterward.
As Allah says in the Quran (Surah Al-Mumtahanah 60:8):
“Allah does not forbid you from being kind and just to those who have not fought you because of your religion and have not driven you from your homes. Indeed, Allah loves those who are just.”