I first read this book 30 years ago. Montross was a US Marine in WW1. He became a journalist after that. Wrote the book after WW2, and the second edition included the Korean War.
It’s a good introduction to war in Western Europe, from the time of the ancient Greeks onward. US Wars are included, as well as South American wars of liberation (but not enough, IMO).
It’s Eurocentric, although the Mongols and initial conquests of the Arabs are covered. It does a good job with the usual suspects of Greece, Rome, the Crusades, but also stuff not usually well covered like Byzantium, the high Middle Ages, and the 30 Years War.
Written in a very easy to understand way. Probably 10th grade level or so.
It’s a good addition to someone’s library for the military section, and really a good starting spot, but it can’t serve as the single book on war in a library due to things not covered