What are the principles and rules, which Muslims have to abide when war is inevitable?

Principles of Jihad (holy struggle) that are stated precisely in the Quran:

1. The principle of the justified war: The reason for war given in the Quran is the attack and the oppression of an enemy. If an enemy invades a Muslims country, forces them to emigrate, threatens the security of life, property, and chastity; in that case war becomes obligation upon the people of that country.

According to Quran, the reason for fighting against the enemy must be rational and just. The religion Islam that does not recognize the wars of invasion, aggression, and occupation; has allowed a war only if it is for preventative reasons such as providing the security of life, property; protecting rights and liberties; suppressing the enemy attacks. In addition, Islam, considering it legitimate named it as Jihad to differentiate from other wars.

2. The principle of fairness: Fair war principle is effectual when it is engaged in a de facto war. According to this principle, war is only against participating countries. Islam aims at winning people over rather than killing them. For this purpose, invites people to Islam before engaging in war, if it is not accepted, war reparation or Jizyah (per capita tax imposed on able non-Muslim men of military age) is sought from them. It cannot be declared a Jihad without attempting those. It is engaged in a war only after if the enemy refuses to accept the conditions proposed.

A Muslim should always take a step according to this fair war principle. The enemy soldier must be chosen as the only target at shooting. Children, women, elders, handicapped, mentally ill cannot be killed during the war. It cannot be drawn weapons on unarmed ecclesiastics, elders who do not participate in war. Civilians (such as artisans, farmers etc.) who do not participate in war cannot be killed either. (Al-Baqarah Surah, 2:191)

Narrated by Savfan Bin Assal: Messenger of God (peace be upon him) sent me to a war (in sariya). He ordered me: Walk in the way of God, in the name of Him. Fight against the deniers, do not torture your captives, do not steal from the booty, do not kill the children. (Sahih Muslim, Jihad 3:1731, Tirmidhi, 48:1617 Abu Dawud, Jihad 90:2612, 2613)

3. The principle of not going to the extremes in a war: Islam advocates humane values even in war times. Ferocity and brutality of the violence in war becomes more moderate. It accepts the principle of not exceeding limits. It provided necessary penal sanctions for persons who illicitly behave. Islamic law embraced the principle of reciprocity while retaliating against the enemy, going to the extremes is crime.

Quran forbids exceeding limits even in face-to face combat, Fight in Gods cause (in order to exalt His Name) against those who fight against you, but do not exceed the bounds (set by God), for surely God loves not those who exceed the bounds. (Al-Baqarah Surah, 2:190)

As a matter of fact, in another verse, it is stated as, So, whoever attacks you, attack them in like manner as they attacked you. Nevertheless, fear God and remain within the bounds of piety and righteousness, and know that God is with the God-revering, pious. (Al-Baqarah Surah, 2:194)

4. The principle of peace: Islam accepts the principle of retaliating with peace whenever the enemy offers peace proposal. And if they (the enemies) incline to peace, incline to it also, and put your trust in God. Surely, He is the All-Hearing, the All-Knowing. (Al-Anfal Surah, 8:61) And with the message of the following verse announces the truth to the whole world for one thousand and four hundred years peaceful settlement is better... (An-Nisa Surah, 4:128) Finally with the following two verses emphasizes the importance of peace, Then if they desist (from fighting), surely God is All-Forgiving, All-Compassionate (especially towards those who return to Him in repentance.) (Al-Baqarah Surah, 2:192) (But if they persist in causing disorder, continue to) fight against them until there is no longer disorder rooted in rebellion against God, and the religion (the right for worship and the authority to order the way of life is recognized) for God. However, if they desist, then there is no hostility except to the wrongdoers. (Al-Baqarah Surah, 2:193)

5. The principle of treating captives kindly: Islam orders to treat captives kindly and well. Muslims are responsible for feeding their captives. They fulfill this duty for the sake of God. (Al-Baqarah Surah,2:177; Anfal Surah, 8:69,70,71; Muhammad Surah, 47:4; Al-Insan Surah, 76:8,9,10,11,12)