From OED:

Methodology: A system of methods used in a particular area of study or activity:

As someone who purports to do research on Medieval History, I am surprised that you do not know that.

Methodology in my field which is physical sciences will be quite different, and includes but is not limited to an analysis of the statistical significance of any conclusions.

As to Stevenson, that is the first indication that I have had of him, and based upon the review and the selected quotes I am not favourably impressed. Phrases like "disgusting establishment toady" come to mind, but I don't know him he may not really be like that, as the review inevitably presents quotes out of context.

The GUEST above puts it well. Historians are useful for assembling facts, but their opinions based on these facts are no more valid than those of anybody else. They can argue that people thought the war worthwhile, although I don't see how they have assembled a large enough database to establish that. But even if they do, it doesn't make it true. The parallel with ISIS is apt.