For starters, the Crusades to the East were in every way defensive wars. They were a direct response to Muslim aggression—an attempt to turn back or defend against Muslim conquests of Christian lands.
Christians in the eleventh century were not paranoid fanatics. Muslims really were gunning for them. While Muslims can be peaceful, Islam was born in war and grew the same way. From the time of Mohammed, the means of Muslim expansion was always the sword. Muslim thought divides the world into two spheres, the Abode of Islam and the Abode of War. Christianity—and for that matter any other non-Muslim religion—has no abode. Christians and Jews can be tolerated within a Muslim state under Muslim rule. But, in traditional Islam, Christian and Jewish states must be destroyed and their lands conquered. When Mohammed was waging war against Mecca in the seventh century, Christianity was the dominant religion of power and wealth. As the faith of the Roman Empire, it spanned the entire Mediterranean, including the Middle East, where it was born. The Christian world, therefore, was a prime target for the earliest caliphs, and it would remain so for Muslim leaders for the next thousand years.
With enormous energy, the warriors of Islam struck out against the Christians shortly after Mohammed’s death. They were extremely successful. Palestine, Syria, and Egypt—once the most heavily Christian areas in the world—quickly succumbed. By the eighth century, Muslim armies had conquered all of Christian North Africa and Spain. In the eleventh century, the Seljuk Turks conquered Asia Minor (modern Turkey), which had been Christian since the time of St. Paul. The old Roman Empire, known to modern historians as the Byzantine Empire, was reduced to little more than Greece. In desperation, the emperor in Constantinople sent word to the Christians of western Europe asking them to aid their brothers and sisters in the East.
That is what gave birth to the Crusades. They were not the brainchild of an ambitious pope or rapacious knights but a response to more than four centuries of conquests in which Muslims had already captured two-thirds of the old Christian world. At some point, Christianity as a faith and a culture had to defend itself or be subsumed by Islam. The Crusades were that defense.