The 1851 book "The Fifteen Decisive Battles of the World"[0] and its ilk is likely what the author refers to when talking of "decisive battles where popularised in the 19th century".
It is pretty obvious that single battles hardly determine long war outcomes, but it's a pretty attractive idea that a kingdom can be lost for want of a nail and all that.
[0] https://en.wikipedia.org/wiki/The_Fifteen_Decisive_Battles_o...