Steven Runciman concludes the final volume (Volume 3) of his A History of the Crusades with this judgement on the two centuries of the crusades:
“The triumphs of the Crusade were the triumphs of faith. But faith without wisdom is a dangerous thing. By the inexorable laws of history the whole world pays for the crimes and follies of each of its citizens. In the long sequence of interaction and fusion between Orient and Occident out of which our civilization has grown, the Crusades were a tragic and destructive episode. The historian as he gazes back across the centuries at their gallant story must find his admiration overcast by sorrow at the witness that it bears to the limitations of human nature. There was so much courage and so little honor, so much devotion and so little understanding. High ideals were besmirched by cruelty and greed, enterprise and endurance by a blind and narrow self-righteousness; and the Holy War itself is nothing more than a long act of intolerance in the name of God, which is the sin against the Holy Ghost.”
In geopolitical terms, there is another way of judging the crusades. The massive warfare and colonization attempts during the period of the crusades revitalized the Western civilization which had been slumbering since the fall of the Roman Empire in the fifth century. The crusades enabled the West to develop an ideology, it gave them a cause to fight for, it made them aware of their own weaknesses and the strengths of their rivals, and eventually, the hard lessons learned from the crusades enabled the West to develop into a world power.