A religious war is a war caused by religious differences. It can involve one state with an established religion against another state with a different religion or a different sect within the same religion, or a religiously motivated group attempting to spread its faith by violence, or to suppress another group because of its religious beliefs or practices. The Muslim Conquests, the French Wars of Religion, the Crusades, and the Reconquista are frequently cited historical examples.
The Muslim concept of Jihad, or Holy War was set down in the 7th Century. Saint Augustine is credited as being the first to detail a "Just War" theory within Christianity, whereby war is justifiable on religious grounds. Saint Thomas Aquinas elaborated on these criteria and his writings were used by the Roman Catholic Church to regulate the actions of European countries.
Many wars that are not religious wars often still include elements of religion, such as priests blessing battleships. Differences in religion can further inflame a war being fought for other reasons. Historically, temples have been destroyed to weaken the morale of the opponent, even when the war itself is not being waged over religious ideals.
In modern times religious designations are sometimes used as shorthand for cultural and historical differences between combatants, giving the impression that the conflict is primarily about religious differences. For example, The Troubles in Northern Ireland are frequently seen as a conflict between Catholics and Protestants. However, the more fundamental cause is the attachment of Northern Ireland to either the Republic of Ireland or the United Kingdom. As the native Irish were mostly Catholic, and the later English-sponsored immigrants mainly Protestant, the terms become shorthand for the two cultures. It cannot be denied that religion does play a part in the conflict, since churches are used as organizing points for demonstrations, and Protestants are far more likely to oppose union with the Catholic-dominated Republic, however, religious differences were not the overriding cause of the conflict.
The cross has been illustrated quite often upon crests of shields, in the plating of armor or even as engravings upon weapons.
Those who fought in the name of God were recognized as Milites Christi, warriors or knights of Christ. Christian fighters believed that victory was achieved through divine intervention or aid from God, and took great pride in their beliefs. These blessed warriors pursued opposing armies and the heretic religions and cults of the time, and were highly admired by the Church and the State. Often, these enemies would be one and the same, such as the Lombard Legions, which were portrayed as a common enemy of Rome and a satanic Pagan tribe as well.
The ideals and duties of religion were used as tools to legitimize warfare. Religion essentially gave the armies an excuse for their conquests under the guise of "Christianization", but their holy conquests turned out to be a long, violent series of raids for territorial expansion, trade and world-wide dominance, in the name of God.
Main article: Crusade
The Crusades were a series of military campaigns—usually sanctioned by the Papacy—that took place during the 11th through 13th centuries in response to the Muslim Conquests. Originally, the goal was to recapture Jerusalem and the Holy Land from the Muslims, and support the besieged Christian Byzantine Empire against the Muslim Seljuq expansion into Asia Minor and Europe proper. Later, Crusades were launched against other targets, either for religious reasons, such as the Albigensian Crusade, the Northern Crusades, or because of political conflict, such as the Aragonese Crusade. In 1095, at the Council of Clermont, Pope Urban II raised the level of war from bellum iustum ("just war"), to bellum sacrum.
Main article: French Wars of Religion
In 16th Century France there was a succession of wars between Roman Catholics and Protestants (Hugenots primarily). These series of wars were known as the Wars of Religion.
Main article: Thirty Years War
In the first half of the 17th century, the German states, Scandinavia (Sweden, primarily) and Poland were beset by religious warfare. Roman Catholicism and Calvinism figured in the opposing sides of this conflict, though Catholic France did take the side of the Protestants but purely for political reasons.
Jihad - See also: Islamic military jurisprudence
Jihad means "to strive or struggle" in the way of God, and is sometimes referred to as the sixth pillar of Islam, although it has no official status. In the West, Jihad is often understood as "Holy War", but it has a broader meaning in Islamic theology. It can be striving to lead a good Muslim life, praying and fasting regularly, being an attentive and faithful spouse and parent or working hard to spread the message of Islam. While there have been mujtahids (Islamic scholars) who have argued that Jihad is not supposed to include aggressive warfare, they have written their treatises in places such as Syria, Eastern Anatolia and Mesopotamia, all of which were wrenched violently from the Persian or Byzantine Empires.
Islamic scholars have different opinions on Jihad, but there is a consensus that armed struggle against persecution and oppression, defined as the rule of any authority counter to the principles of Islam, will always continue.
Main article: Milhemet Mitzvah
In the Jewish religion, the expression Milhemet Mitzvah (Hebrew: מלחמת מצווה, "commandment war") refers to a war that is both obligatory for all Jews (men and women) and limited to within the borders of the land of Israel. The geographical limits of Israel, and therefore of this religious war, are detailed in the Tanakh, the Hebrew Bible, especially Numbers 34:1-15 and Ezekiel 47:13-20. The relevance of this concept to the contemporary State of Israel is debated. There is no reference to Milhemet Mitzvah in official documents from the Israeli government or defense forces.
The Holy Land of Islam is Saudi Arabia, Makkah and Medina.
Main articles: State Shinto, National Spiritual Mobilization Movement, and League of Diet Members Believing the Objectives of the Holy War
During the first part of the Showa era, imperial propaganda, turning to the Empire of Japan's spiritual capital and to maintain fighting spirit, called the Second Sino-Japanese War a holy war (Seisen). This propaganda was based on hakko ichiu, the traditional belief that imperial rule had been divinely ordained to expand until it united the "eight corners of the world".
In Sikhism, a Holy War is only eligible when all means of peace have been tried and failed. The early Sikhs of the 15-1600's fought off many Mughal Islamic Invaders, as well as Hindu Rajputs. Sikhs are only allowed to fight for the well being of righteousness, even stated by their 10th guru, Guru Gobind Singh. ""When all other means have failed, It is but lawful to take to the sword."
Main article: Saxon Wars
The Saxon Wars have been described as a religious war.
During the period of the explicitly statist Zoroastrianism of the Sassanid era, it was political custom for wars to be proclaimed for the faith rather than the state. In this sense, all wars which the Sassanid Persian Empire engaged in were religious wars.