Our knowledge of the Crusades has been expanding rapidly in the past few decades. A good starting point is to read some of the numerous works of Dr. Riley-Smith.
Here is a link to a First Things Article in which Riley-Smith explains what the Crusades were:
A good examination of myths about the crusades, linked below, by Thomas F. Madden, one of the foremost historians of the Fourth Crusade.
His “A Concise History of the Crusades” is a must read for anyone interested in this period in history:
The Crusades were a tardy, and defensive, response to militant Islam by the Catholic states of the West. By bringing Western military power against Islam the fall of Constantinople to the Turks was delayed until 1453. The Byzantine Empire had suffered a severe defeat at the battle of Manzikert at the hands of the Turks in 1071. They were no longer able to hold the line in the East against Islam and were desperate for military aid from the West. Absent the Crusades I doubt if Constantinople would have survived much beyond 1150. This would have led to Islam taking over the Balkans three centuries before it did historically. These three centuries were crucial in that by the time the Turks marched against Vienna in 1529 the West was already beginning to surpass Islam technologically. Vienna besieged in 1229 might have been the beginning of a process that would have seen the conquest of Europe by Islam.
It is important that we understand this history, because the West today is under siege again by Islam and too many people in the West, blinded by Leftism and a fashionable scorn, if not outright hatred for, Christianity, paint a false picture of the past and the present in which Islam is a religion of peace and Christians are the evil agressors. Such madness will kill us all if are not constantly reminding people of the truth.