The Crusades, those times of war and atrocity across Europe and the Levant that defined much of the brutality of the Middle Ages, are most famous as conflicts between Christians and Muslims. Indeed, those two religious groups contributed the largest part of the soldiers and suffered the highest casualties of the Crusades, though they were not the only ones to fight. Jews experienced much hardship during that troubled period, such that they wouldn't see the same level of violence and persecution until the rise of fascism in Europe in the early 20th century.
The Crusades were a politically complicated series of wars and grand, social gestures. No one thing precipitated the First Crusade launched by Pope Urban II. Rather, it was a natural result of various shifts in the social and military power of Christian entities throughout Europe, Africa and Asia. Islam had been steadily growing in influence for centuries, threatening to supplant the hegemony of the forces of the Holy Roman Empire, the Byzantine Empire and the Papal States. When Muslim empires began to capture lands in the Middle and Near East from Byzantium, Christian powers conspired to launch a series of response attacks in a bid to cut their losses.
This was a huge undertaking, socially speaking. The religious charge that compelled so many Western Christians to join the Crusades can be seen as a sort of marketing push. The campaign to reclaim Jerusalem, a holy city of dubious strategical worth, came as an additional goal long after the First Crusade was planned. By making the Crusades about spiritual rather than political concerns, the Christian forces had locked themselves in what would end up being a tireless back-and-forth of bloody warfare.
A byproduct of this religious violence was intense anti-semitism. Jewish communities throughout Europe experienced persecution and even extermination as sentiments flared against any non-Christian peoples. It became popular for states to enact anti-Jewish laws, most notably the rulings of Pope Innocent III. This widespread hatred directed at Jews led to Jewish support for Muslim forces in Israel.
Following the fall of the Western Roman Empire, Jews slowly returned to their homeland and established settlements. Cities such as Haifa and Tiberias saw significant military support from Jewish armies during the First Crusade, while Jews notably allied themselves with the Muslim occupiers during the assault on Jerusalem. For their trouble, they were slaughtered, enslaved and ransomed. Many important works of Jewish literature barely escaped the aftermath.
The horrors of the Crusades created a rift between Jews and Christians that has only really begun to heal in the modern day. The social isolation that followed the persecution of Jews in the Middle Ages paved the way for the pogroms of Czarist Russia and the concentration camps of Nazi Germany, just as the great battles of the Crusades set the stage for modern warfare in the Middle East.