In the 620s AD, a new religion was born in present-day Saudi Arabia. Unlike Christianity, which was spread purely by persuasion and apologetics for 600 years up until then, this new religion — Islam — was, from the beginning, spread by warriors. Everywhere they went, followers of Muhammad would attack, attack, and attack. Most of the Middle East was Christianized by that point — in a wide swath stretching from Arabia *all* the way to Europe, Christianity was the dominant religion, and northern Arabia in particular was part of the Byzantine Empire, which Eastern Orthodoxy was the state church of. When Islamists invaded, they gave innocent Christians three options: convert to Islam, pay a high jizya tax, or die. 300 years after Roman persecution ended, Islamic persecution had begun.
These attacks by Islamists against the Christian world continued for 400 years after this. From the 600s to the 1000s AD, they attacked North Africa, Egypt (strongly Christian at the time), and Spain. Meanwhile, in the Middle East, persecution of Christians became all the more rampant. It was at this point, during a period of unprecedented aggression by the Rashidun, Umayyad, and Fatimid Caliphates, that the Church decided to send a message. They pushed back against al-Andalus and temporarily took what is now Israel back from the Islamists.
The Crusades, as the response to 400 years of jihad would come to be called, were a purely reactionary response to 400+ years of Islamist aggression. Yet what do Islamists say whenever people claim that terrorism in the name of Islam is more rampant than it is in the name of any other faith? They use the crusades as a tu quoque argument to discredit those who make it. Not only is "what about the crusades" a fallacy to begin with — tu quoque is precisely that, a fallacy — but it also expresses total ignorance of the Crusades' reactionary nature.
Until academics become forced at gunpoint to include these additional 400 years of Islamist history preceding the Crusades in textbooks about the Crusades — after all, 400 years seems to also be a magic number in the Bible as far as divine judgment is concerned — expect these arguments that are nothing but pure evil to continue. People must be forced to admit this history at all costs, even if a law is passed making it illegal for professors to leave it out, why? Because ignorance of history makes people doomed to repeat it.