The First Crusade began towards the end of the 11th century. On their way towards the Holy Land, the Crusaders passed the Rhine River and several Jewish villages there. They passed through around the time of the holiday of Shavuot, the holiday coming next week, and slaughtered every Jew in the villages they passed who would not convert to Christianity. This event was marked in the Ashkenazic liturgy by saying Av Harachamim on the Shabbatot before Shavuot and before Tisha B'Av (that is the original practice, as other massacres took place in other communities, many marked by saying Av Harachim on the anniversaries of those massacres, the practice became to expand the occasions on which it is said in most communities).
A few notes connecting that event to our times. One, just as the Crusaders were motivated by religious passion, Muslim jihadis do so today. Two, just as the Crusaders offered a particular appeal to the criminals of their time, so does today's call for avenging the honor of Islam. Three, just as participating in the Crusades promised one forgiveness for all sins and thus a ticket to heaven, today's avengers of the honor of Islam are promised 72 virgins in paradise.
A few more notes. While the attitudes towards non-believers in Christianity persisted for centuries afterwards, new theology did eventually take root in Christianity. Today, the theological tenets that propelled the Crusades among Christians nine centuries ago are popular across a broad segment of Islam. Many outside of Islam argue that these tenets are an immutable aspect of Islam and that we are therefore at war with all of Islam. My question to proponents of that notion is what about Islam today could not have been said about Christianity of the 11th century and thus can not be overcome the way that the Crusading theology of Christianity was superceded by Nostra Aetate? This is not to advocate any let up against those who profess an Islamic version of the Crusaders' theology, only to recognize the difference between practicing the five pillars and professing that theology.
Along a different strand of thought, "jihad" is term of honor to many Muslims. Does the fact that those avenging the honor of Islam call their action "jihad" mean that we should dignify them by using their preferred term as well? As I showed above, their actions have substantial parallels with the Crusades. Further, the term "Crusader" is derogatory term for Muslims, recall bin Laden calling those attacking him "Crusaders and Jews." Why not call them Crusaders, or to specify their cause, Islamic Crusaders?