A comprehensive history of armed conflict would require a book of some length and here we are attempting to cover the topic in a short space, meaning that some details are left out and general themes presented instead. The simplest way to do this is to categorise the historical record through major trends, and so allowing a general understanding at the expense of omitting detail. It is also the case that historical categories, while being clearly separated, have a tendency to overlap and as with all aspects of human history, characteristics of one era may be found in another.