If the reader is seeking a comprehensive review of all four Crusades (or more depending on the historian doing the counting), he would do better looking for another tome, because Belloc concentrates his efforts on the First Crusade, which was arguably the most successful since the result was the capture of a majority of the Holy Land and Jerusalem for nearly a century, a feat never later repeated.
Writing in 1937, Belloc manages a prescient guess at the import of the events he relates, warning that Islam would once again rise to confront the European, Christian states. This was before the rise of Islamic fundamentalism in the 1970s, before the end of Europe's domination of the Arabic world through colonization, and a decade before Israel was carved out of Palestine after the end of World War II.
Belloc's first achievement is to describe the state of feudal Europe at the beginning of the second millennium of Christ, showing the tenuous hold of the various kings over their nominal subjects and describing how the real secular powers, the feudal dukes and counts, had become so after inheriting their domains through the centuries from the former Roman governors who held the lands as the empire crumbled.
The feudal mindset was crucial to the initial success and eventual failure of the crusades to accomplish their goal of holding the Holy Lands and denying them to Islam. The inability to coordinate militarily with others of near-equal rank, the jockeying for power, and the lack of a strategic view all eventually came to mean the loss of Jerusalem. Belloc takes great pains to describe how various military decisions contributed to both wins and losses for the Crusaders and shows how the decision not to take Damascus when it was available at the start of the Crusade eventually resulted in the final defeat at Hattin.
Contrary to current myths, the Crusades were not a simple bloody campaign by Christian knights against the peaceful Muslims of the Middle East. The truth is always more complex than the one-sentence explanation. Rather, the Crusades were an attempt to stop the marauding attacks on unarmed pilgrims making religious visits to the Holy Land. It also shored up the Byzantine Empire after the terrible defeat at Manzikert in 1071 at the hands of the Asian Muslim "Turks". The Crusades freed native Christians of the Middle East from their oppressive Muslim masters.
Did the Crusaders do things that would seem un-Christian to us? Does the seeming bloodthirstiness of both sides make us uneasy? Yes to both questions, but Belloc's gift is to make his readers understand that not everything can be judged fairly through 20th century eyes. We must understand the people of the time and their thinking to understand the why's and how's.
Bottom Line : The Crusades is a good book. I was surprised at its emphasis on military and political matters over the religious issues, but it was a pleasant surprise, because the topics became fascinating. I do wish the book contained more maps and diagrams, especially of the battles since my ability to diagram the events in my mind is limited. And the unflattering references to Islam and Muslims by Belloc sometimes made me cringe as I imagine how they would play in these politically correct times. But overall, the book accomplishes its goal of explaining the who, what, where, when, why, and how of the First Crusade.