1. To engage in a crusade for a certain cause or person; SYN. fight, campaign, push, agitate.
2. To go on a crusade; fight a holy war.

ETYM French croisade, from Pr. crozada, or Sp cruzada, or Italian crociata, from a verb signifying to take the cross, mark one's self with a cross, from Latin crux cross; or possibly taken into English directly from Pr. Related to Croisade, Crosado, and see Cross.
1. Any one of the military expeditions undertaken by Christian powers, in the 11th, 12th, and 13th centuries, for the recovery of the Holy Land from the Mohammedans.
2. Any enterprise undertaken with zeal and enthusiasm.
3. A Portuguese coin.
European war against non-Christians and heretics, sanctioned by the pope; in particular, the Crusades, a series of wars 1096–1291 undertaken by European rulers to recover Palestine from the Muslims. Motivated by religious zeal, the desire for land, and the trading ambitions of the major Italian cities, the Crusades were varied in their aims and effects.
The Crusades ostensibly began to ensure the safety of pilgrims visiting the Holy Sepulcher and to establish Christian rule in Palestine and they continued for over two centuries, with hardly a decade passing without one or more expeditions. Later they were extended to include most of the Middle East and attacks were directed against Egypt and even Constantinople.
The first Crusades.
In Palestine the mild rule of the first Muslim conquerors had for centuries allowed a Christian protectorate (first established under Charlemagne) to exist in Jerusalem, and Christian pilgrims were allowed to come and go quite freely. However, peaceful coexistence was shattered 1010 when the caliph Hakim destroyed the sanctuary. After 1071, the Saracens were driven out by the Seljuk Turks and Christian pilgrimage became difficult and dangerous. In 1095 Pope Urban II appealed for protection for the pilgrims, giving the turbulent feudal knights of Europe a new outlet for their energies and the first Crusades were launched.
1095–1148.
In 1095 several undisciplined hosts, such as those of Walter the Penniless and Peter the Hermit, set out for the East, but perished on the way. A more serious expedition was mounted 1096–97: a great army under Godfrey de Bouillon, Bohemund of Otranto, and other leaders, fought its way through Asia Minor, taking Antioch 1098, and Jerusalem 1099. A Christian kingdom was established, with Godfrey as its first head, his brother Baldwin as Count of Edessa (Upper Mesopotamia), and Bohemund ruling at Antioch. Godfrey died 1100 and was succeeded by Baldwin. During the next half-century, in spite of reinforcements, including fleets from Genoa, Norway, and Venice, the Christians in Syria were hard-pressed and the orders of Knights Hospitallers and Knights Templars were formed to assist in the defense of Jerusalem. Edessa was lost 1144, and the Second Crusade, under Louis VII of France and Conrad III of Germany (1146–48), ended disastrously. Its failure for a time discouraged any similar ventures, while Muslim pressure.
Increased on all sides.
Saladin’s conquests and European reaction.
The crusading spirit was revived toward the end of the 12th century by the conquests of Saladin, Sultan of Egypt. Having captured Damascus 1174 and Aleppo 1183, he swept down through Galilee with an immense force, defeated a Christian army at the Horns of Hattin, and took Jerusalem Oct 1187. The news was received in Europe with consternation and rage. Several fresh expeditions were mounted, of which the most important was that led by Philip of France, Frederick of Germany, and Richard I of England 1189. The Germans went through Asia Minor, losing their emperor on the way; the French and English went by sea to Acre, which had already been besieged for nearly two years by Guy de Lusignan. Richard distinguished himself in the capture of the city, but quarreled with his allies, who left him to carry on the war alone. After a year of brilliant but useless exploits, he made a truce with Saladin, and returned to Europe.
13th-century crusades.
Another crusade, starting from Venice 1202, became involved in Venetian and Byzantine intrigues, and instead of reaching Jerusalem, assisted the deposed Isaac Angelus to regain the Greek throne; a few months later Constantinople was stormed and sacked by the crusaders, and a Latin empire established there under Baldwin of Flanders 1204. Perhaps the most tragic of these expeditions was the children’s crusade 1212: several thousand children were allowed to go on a crusade, many dying on their way from France and Germany to the Italian coast. The rest embarked, but those who reached Alexandria were captured and sold into slavery. Andrew of Hungary led a failed crusade 1217–21 against the Muslims in Egypt.
Frederick II undertook a more successful crusade 1228. He regained Jerusalem and the south of Palestine by diplomacy, not fighting, and it remained in Christian hands until 1244. The crusade of Louis IX of France (St Louis) 1249 was, like that of 1217, directed against Egypt, and proved even more disastrous. Louis was captured with the greater part of his army, and had to pay 800,000 pieces of gold as a ransom. Even after this, he headed another crusade 1270, but died at Carthage. Prince Edward of England (afterwards Edward I), a few months later led his own followers to Acre, but achieved no results.
Diminishing enthusiasm.
Understandably, after so many failures and only temporary successes, the enthusiasm for crusades died down. In the 14th century several Crusades were mounted against the Ottoman Turks, but these were more defensive movements designed to stop the rapid advances of the Turks who were encroaching on the eastern borders of Europe. Several popes continued to preach about the need for a united Christian holy war against the infidels, but to no avail. Even when Constantinople was captured by Mohammed II 1453, Pius II failed to raise a crusade for its recovery. The Templars had been suppressed, but the Hospitallers, at Rhodes and afterwards at Malta, continued to be a bulwark against Turkish advance in the Mediterranean.
Failure and benefits.
Although the crusades failed to achieve the spiritual objects for which they were intended and led to much needless slaughter, they benefited Europe indirectly in a number of ways. Trade between Europe and Asia Minor was greatly stimulated; the merchants and mariners of the Mediterranean, especially of Venice and Genoa, found the demand for their shipping increased manifold, both for the transport of armies and for the bringing of new and rare commodities from east European craftsmen, and crusaders learned valuable lessons from Saracen skills in art and in war. Sugar, cotton, and many other articles now of everyday use first became known in Europe through the crusades. The cultural contacts which were established between Europe and the East in this period had a stimulating effect on the medieval learning of Europe, and to some extent anticipated and paved the way for the Renaissance.

Any of the more or less continuous military expeditions in the 11-13th centuries when Christian powers of Europe tried to recapture the Holy Land from the Muslims.