Wartime sexual violence
Definition of wartime sexual violence: There is no consensus definition of wartime sexual violence, as there is variance in what forms of violence are included in the definition and variance in which violence is considered conflict-related. The terms rape, sexual assault and sexual violence are frequently used interchangeably. The Explanatory Note of the Rome Statute, which binds the International Criminal Court, defines rape as follows:

The perpetrator invaded the body of a person by conduct resulting in penetration, however slight, of any part of the body of the victim or of the perpetrator with a sexual organ, or of the anal or genital opening of the victim with any object or any other part of the body.
and

The invasion was committed by force, or by threat of force or coercion, such as that caused by fear of violence, duress, detention, psychological oppression or abuse of power, against such person or another person, or by taking advantage of a coercive environment, or the invasion was committed against a person incapable of giving genuine consent.
The concept of "invasion" is intended to be broad enough to be gender-neutral and the definition is understood to include situations where the victim may be incapable of giving genuine consent if affected by natural, induced or age-related incapacity.
A prominent data-set on conflict-related sexual violence, Sexual Violence in Armed Conflict (SVAC), builds on the ICC definition and covers seven forms of violence: "(a) rape, (b) sexual slavery, (c) forced prostitution, (d) forced pregnancy, (e) forced sterilization/abortion, (f) sexual mutilation, and (g) sexual torture." The data-set defines conflict-related sexual violence as the sexual violence committed by "armed actors (specifically, state militaries, rebel groups, and progovernment militias) during periods of conflict or immediately postconflict," thus excluding sexual violence by civilians. More expansive definitions may define wartime sexual violence as being committed even by civilians if the conflict creates a sense of impunity.
In 2009, the UN established a mandate and adopted SCR 1888 resolution 2009 to tackle conflict-related sexual violence (CRSV) as a peace and security issue and related violations. CRSV refers to rape, sexual slavery, forced prostitution, forced pregnancy, forced abortion, enforced sterilization, forced marriage, trafficking in persons when committed in situations of conflict for the purpose of sexual violence/exploitation and any other form of sexual violence of comparable gravity perpetrated against women, men, girls or boys that is directly or indirectly linked to a conflict.

History of laws against sexual assault during war: Prosecution of rapists in war crime tribunals was rare prior to the late 1990s. Kelly Dawn Askin, the senior legal officer at the Open Society Foundation, argued that the lack of explicit recognition of war rape in international law or applicable humanitarian law can not be used as a defense by a perpetrator of war rape. Laws and customs of war prohibit offenses such as "inhuman treatment" or "indecent assaults", adding to this domestic military codes and domestic civil codes (national law) may make sexual assault a crime.
In 1999, humanitarian law concerned the maltreatment of civilians and "any devastation not justified by military necessity".

Classical period: The ancient Greeks considered war rape of women "socially acceptable behavior well within the rules of warfare", and warriors considered the conquered women "legitimate booty, useful as wives, concubines, slave labor, or battle-camp trophy". One of the first references to the "laws of war", or "traditions of war" was by Cicero, who urged soldiers to observe the rules of war, since obeying the regulations separated the "men" from the "brutes". Conquering the riches and property of an enemy was regarded as legitimate reason for war in itself. Women were included with "property", since they were considered under the lawful ownership of a man, whether a father, husband, slave master, or guardian. In this context, the rape of a woman was considered a property crime committed against the man who owned the woman.

Pre-modern Europe: In the Middle Ages, the Catholic Church sought to prevent rape during feudal warfare through the institution of Peace and Truce of God (first proclaimed in 989) which discouraged soldiers from attacking women and civilians in general and through the propagation of a Christianized version of chivalry ideal of a knight who protected innocents and did not engage in lawlessness.
In 1159, John of Salisbury wrote Policraticus in an attempt to regulate the conduct of armies engaged in "justifiable" wars. Salisbury believed that acts of theft and "rapine" (property crimes) should receive the most severe punishment, but also believed that obeying a superior's commands whether legal or illegal, moral or immoral, was the ultimate duty of the soldier.
Rape and pillage were prohibited by some army codes as early as the 14th century because of the tendency to create strong hostility in civilian populations and the detrimental effects to army discipline. Despite early efforts to systematize the laws of war, rape continued to be a problem in the 15th and 16th centuries. The influential writer Francisco de Vitoria stood for a gradual emergence of the notion that glory or conquest were not necessarily acceptable reasons to start a war. The jurist Alberico Gentili insisted that all women, including female combatants, should be spared from sexual assault in wartime.
It is suggested that one reason for the prevalence of war rape was that at the time, military circles supported the notion that all persons, including women and children, were still the enemy, with the belligerent having conquering rights over them. In the late Middle Ages, the laws of war even considered war rape as an indication of a man's success in the battlefield and "opportunities to rape and loot were among the few advantages open to... soldiers, who were paid with great irregularity by their leaders.... triumph over women by rape became a way to measure victory, part of a soldier's proof of masculinity and success, a tangible reward for services rendered.... an actual reward of war".
During this period in history, war rape took place not necessarily as a conscious effort of war to terrorize the enemy, but rather as earned compensation for winning a war. There is little evidence to suggest that superiors regularly ordered subordinates to commit acts of rape. Throughout this period of history, war became more regulated, specific, and regimented. The first formal prosecution for war crimes did not take place until the late Middle Ages.

Early modern Europe: Hugo Grotius, considered the father of the law of nations and the first to conduct a comprehensive work on systematizing the international laws of war, De jure belli ac pacis (1625), concluded that rape (phrased as "the violation of women") "should not go unpunished in war any more than in peace", rejecting the view that it is permissible to inflict damage or harm "on anything belonging to the enemy":

The violation of women [in wartime] is variously regarded as permitted or not. Those who allow it consider only the injury done to the person, which they hold it agreeable to the law of arms to inflict on anything belonging to the enemy. The other opinion is better: it takes into account not only the injury but also the act of unbridled lust and concludes that something pertaining to neither safety nor punishment should be no more lawful in war than in peace. This latter view is not the law of all nations, but it is the law of the more respectable ones.
Emmerich van Vattel emerged as an influential figure when he pleaded for the immunity of civilians against the ravages of war, considering men and women civilians as non-combatants.
In the late 18th century and 19th century, treaties and war codes started to include vague provisions for the protection of women: The Treaty of Amity and Commerce (1785) specified that in case of war "women and children.... shall not be molested in their persons". Article 20 of the Order No. 20 (1847), a supplement to the US Rules and Articles of war, listed the following as severely punishable "Assassination, murder, malicious stabbing or maiming, rape". The Declaration of Brussels (1874) stated that the "honours and rights of the family.... should be respected" (article 38).
In the 19th century, the treatment of soldiers, prisoners, the wounded, and civilians improved when core elements of the laws of war were put in place by nations who were signatories to treaties. However, while the customs of war mandated more humane treatment of soldiers and civilians, new weapons and advanced technology increased destruction and altered the methods of war.
The Lieber Code (1863) was the first codification of the international customary laws of land war and an important step towards humanitarian law. The Lieber Code emphasized protection of civilians and stated that "all rape... [is] prohibited under the penalty of death", which was the first prohibition of rape in customary humanitarian law.

20th century: During the 20th century, international legal procedures attempted to prevent and prosecute perpetrators of war rape. Similarly, individual states developed laws pertaining to war rape's victims and perpetrators.
The prohibition of rape was excluded from the Geneva Conventions and was deliberately left vague by the Hague Conventions. Article 46 of the Hague Conventions of 1899 and 1907 regarding Land Warfare required only that "[f]amily honour and rights [and] the lives of persons... must be respected" by the occupying powers.