The Crusades were a series of religious wars initiated, supported, and sometimes directed by the Latin Church in the medieval period. The best known of these military expeditions are those to the Holy Land in the period between 1095 and 1291 that were intended to conquer Jerusalem and its surrounding area from Muslim rule. Beginning with the First Crusade, which resulted in the conquest of Jerusalem in 1099, dozens of military campaigns were organised, providing a focal point of European history for centuries. Crusading declined rapidly after the 15th century.

In 1095, Pope Urban II proclaimed the first expedition at the Council of Clermont. He encouraged military support for Byzantine emperor Alexios I Komnenos against the Seljuk Turks and called for an armed pilgrimage to Jerusalem. Across all social strata in western Europe there was an enthusiastic response. Participants came from all over Europe and had a variety of motivations, including religious salvation, satisfying feudal obligations, opportunities for renown, and economic or political advantage. Later expeditions were conducted by generally more organized armies, sometimes led by a king. All were granted papal indulgences. Initial successes established four Crusader states: the County of Edessa; the Principality of Antioch; the Kingdom of Jerusalem; and the County of Tripoli. A European presence remained in the region in some form until the fall of Acre in 1291. After this, no further large military campaigns were organised.

Other church-sanctioned campaigns include crusades against Christians not obeying papal rulings, against the Ottoman Empire, and for political reasons. The struggle between Christians and Muslims in the Iberian Peninsula was proclaimed a crusade in 1123, but eventually became better known as the Reconquista, and only ended in 1492 with the fall of the Muslim Emirate of Granada. From 1147, campaigns in Northern Europe against pagan tribes were considered crusades. In 1199, Pope Innocent III began the practice of proclaiming crusades against what the Latin Church considered heretic Christian communities. Crusades were called against the Cathars in Languedoc and against Bosnia; against the Waldensians in Savoy and the Hussites in Bohemia; and in response to the rise of the Ottoman Empire. Unsanctioned by the church, there were also several popular Crusades.