What do you mean by "Material forces drive history"?

Well, material refers to anything substantial or, more precisely, made of matter. What this person is saying is that it is our interactions with the material world that drive us to do certain things, rather than our interactions with spiritual worlds. This is arguable, but the largest contributing factor to wars of all kinds throughout history have been access to resources, particularly food and water. It doesn't get much more "material" than drinking and eating.

Now religious wars like The Crusades occurred, if taken at face value, as a result of a spiritual decree. In actuality though, the "reclamation" of the Holy Land was again a material concern because even though the land was considered "holy" the whole endeavor was, after all, about occupying space. Land.