Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is well documented, both in ancient and in recent times.The rate of cannibalism increases in nutritionally poor environments as individuals turn to members of their own species as an additional food source. Cannibalism regulates population numbers, whereby resources such as food, shelter and territory become more readily available with the decrease of potential competition. Although it may benefit the individual, it has been shown that the presence of cannibalism decreases the expected survival rate of the whole population and increases the risk of consuming a relative. Other negative effects may include the increased risk of pathogen transmission as the encounter rate of hosts increases. Cannibalism, however, does notas once believedoccur only as a result of extreme food shortage or of artificial/unnatural conditions, but may also occur under natural conditions in a variety of species.Cannibalism is prevalent in aquatic ecosystems, in which up to approximately 90% of the organisms engage in cannibalistic activity at some point in their life-cycle. Cannibalism is not restricted to carnivorous species: it also occurs in herbivores and in detritivores. Sexual cannibalism normally involves the consumption of the male by the female individual before, during or after copulation. Other forms of cannibalism include size-structured cannibalism and intrauterine cannibalism.

Behavioral, physiological and morphological adaptations have evolved to decrease the rate of cannibalism in individual species.

The First Crusade (1096–1099) was the first of a series of religious wars, or Crusades, initiated, supported and at times directed by the Latin Church in the medieval period. The objective was the recovery of the Holy Land from Islamic rule. While Jerusalem had been under Muslim rule for hundreds of years, by the 11th century the Seljuk takeover of the region threatened local Christian populations, pilgrimages from the West, and the Byzantine Empire itself. The earliest initiative for the First Crusade began in 1095 when Byzantine emperor Alexios I Komnenos requested military support from the Council of Piacenza in the empire's conflict with the Seljuk-led Turks. This was followed later in the year by the Council of Clermont, during which Pope Urban II supported the Byzantine request for military assistance and also urged faithful Christians to undertake an armed pilgrimage to Jerusalem.

This call was met with an enthusiastic popular response across all social classes in western Europe. Mobs of predominantly poor Christians numbering in the thousands, led by Peter the Hermit, a French priest, were the first to respond. What has become known as the People's Crusade passed through Germany and indulged in wide-ranging anti-Jewish activities, including the Rhineland massacres. On leaving Byzantine-controlled territory in Anatolia, they were annihilated in a Turkish ambush led by the Seljuk Kilij Arslan at the Battle of Civetot in October 1096.

In what has become known as the Princes' Crusade, members of the high nobility and their followers embarked in late-summer 1096 and arrived at Constantinople between November and April the following year. This was a large feudal host led by notable Western European princes: southern French forces under Raymond IV of Toulouse and Adhemar of Le Puy; men from Upper and Lower Lorraine led by Godfrey of Bouillon and his brother Baldwin of Boulogne; Italo-Norman forces led by Bohemond of Taranto and his nephew Tancred; as well as various contingents consisting of northern French and Flemish forces under Robert Curthose (Robert II of Normandy), Stephen of Blois, Hugh of Vermandois, and Robert II of Flanders. In total and including non-combatants, the forces are estimated to have numbered as many as 100,000.

The crusaders marched into Anatolia. With Kilij Arslan absent, a Frankish attack and Byzantine naval assault during the Siege of Nicea in June 1097 resulted in an initial victory for the crusaders. In July, the crusaders won the Battle of Dorylaeum, fighting Turkish lightly-armoured mounted archers. Next the crusaders marched through Anatolia, suffering casualties from starvation, thirst, and disease. The decisive and bloody Siege of Antioch was fought beginning in 1097 and the city was captured by the crusaders in June 1098. Jerusalem was reached in June 1099 and the Siege of Jerusalem resulted in the city being taken by assault from 7 June to 15 July 1099, during which its defenders were ruthlessly massacred. The Kingdom of Jerusalem was established as a secular state under the rule of Godfrey of Bouillon, who shunned the title of "king". A counterattack was repulsed that year at the Battle of Ascalon, ending the First Crusade. Afterwards the majority of the crusaders returned home.

Four Crusader states were established in the Holy Land: the Kingdom of Jerusalem, the County of Edessa, the Principality of Antioch, and the County of Tripoli. The Crusader presence remained in the region in some form until the loss of the last major Crusader stronghold in the Siege of Acre in 1291. After this loss of all Crusader territory in the Levant, there were no further substantive attempts to recover the Holy Land.