Here is an interesting article from Intercollegiate Studies Institute.

Intro:

The verdict seems unanimous. From presidential speeches to role-playing games, the crusades are depicted as a deplorably violent episode in which thuggish Westerners trundled off, unprovoked, to murder and pillage peace-loving, sophisticated Muslims, laying down patterns of outrageous oppression that would be repeated throughout subsequent history. In many corners of the Western world today, this view is too commonplace and apparently obvious even to be challenged.

But unanimity is not a guarantee of accuracy. What everyone “knows” about the crusades may not, in fact, be true. From the many popular notions about the crusades, let us pick four and see if they bear close examination.

The four myths:

• Myth #1: The crusades represented an unprovoked attack by Western Christians on the Muslim world.
• Myth #2: Western Christians went on crusade because their greed led them to plunder Muslims in order to get rich.
• Myth #3: Crusaders were a cynical lot who did not really believe their own religious propaganda; rather, they had ulterior, materialistic motives.
• Myth #4: The crusades taught Muslims to hate and attack Christians.

Here’s the most obvious thing you should know. The Crusades were defensive actions:

In a.d. 632, Egypt, Palestine, Syria, Asia Minor, North Africa, Spain, France, Italy, and the islands of Sicily, Sardinia, and Corsica were all Christian territories. Inside the boundaries of the Roman Empire, which was still fully functional in the eastern Mediterranean, orthodox Christianity was the official, and overwhelmingly majority, religion. Outside those boundaries were other large Christian communities—not necessarily orthodox and Catholic, but still Christian. Most of the Christian population of Persia, for example, was Nestorian. Certainly there were many Christian communities in Arabia.

By a.d. 732, a century later, Christians had lost Egypt, Palestine, Syria, North Africa, Spain, most of Asia Minor, and southern France. Italy and her associated islands were under threat, and the islands would come under Muslim rule in the next century. The Christian communities of Arabia were entirely destroyed in or shortly after 633, when Jews and Christians alike were expelled from the peninsula.⁶ Those in Persia were under severe pressure. Two-thirds of the formerly Roman Christian world was now ruled by Muslims.

What had happened? Most people actually know the answer, if pressed—though for some reason they do not usually connect the answer with the crusades. The answer is the rise of Islam. Every one of the listed regions was taken, within the space of a hundred years, from Christian control by violence, in the course of military campaigns deliberately designed to expand Muslim territory at the expense of Islam’s neighbors. Nor did this conclude Islam’s program of conquest. The attacks continued, punctuated from time to time by Christian attempts to push back. Charlemagne blocked the Muslim advance in far western Europe in about a.d. 800, but Islamic forces simply shifted their focus and began to island-hop across from North Africa toward Italy and the French coast, attacking the Italian mainland by 837. A confused struggle for control of southern and central Italy continued for the rest of the ninth century and into the tenth. In the hundred years between 850 and 950, Benedictine monks were driven out of ancient monasteries, the Papal States were overrun, and Muslim pirate bases were established along the coast of northern Italy and southern France, from which attacks on the deep inland were launched. Desperate to protect victimized Christians, popes became involved in the tenth and early eleventh centuries in directing the defense of the territory around them.

If you asked me what are the two best books on the Crusades, I would answer God’s Battalions: The Case for the Crusades by Baylor professor Rodney Stark and The Concise History of the Crusades by Professor Thomas F. Madden. If you get this question a lot from atheists, then I recommend you pick these up. Anything by Rodney Stark is useful for Christians, in fact.