Having the words you write be dispassionate and objective doesn't mean you lack an agenda. Everything written ever picks a small set of facts to consider from an effectively infinite set of facts.

The choice of which facts to include, moreover, represents a fairly strong bias; it says that this set of facts is more interesting or important or whatever than most other sets, and enough so that you should ignore all the other sets for the moment and just look at this set.

Which, in the case of this particular book, presumably, along with the countless choices about which facts to include about each battle, also includes at least a) the choice of which 100 battles, and b) the choice that battles of any kind are important enough to talk about for a whole book.

So even if the author tries to stay away from expressing an agenda on the usual axes, I doubt he successfully avoids having one altogether.