I thoroughly enjoyed reading Western Warfare. One of my favourite periods of history to study is Medieval Europe and I like medieval arms and armour. So France's book was a great addition to my limited knowledge on the topic.
France's main angle is to present the centrality of landholding to medieval society and warfare. With no kingdoms in existence local lords and nobles were in charge of their own lands and had to possess the strength and resources to maintain their land. Kings existed but their power was very limited and often they were on equal footing with their wealthiest vassals. Land was divided into tiny fiefdoms and any one noble had control of several areas, often spread out and not adjoining areas. Each noble typically owed vassalage to multiple lords, in a complicated web of alliances. For instance a count could give homage and support to both the King of France and the King of England and think it perfectly normal.
The nobles built castles and fortifications to act as defensive posts, but also as offensive posts to attack a neighbor. Medieval warfare tended to occur on a very small scale, raids and ravage and small skirmishes occurred far more often than large scale pitched battles. In pitched battles mounted troops, the knights and sergeants, always worked best when combined with large numbers of infantry. Sieges were the common tactic employed. Nobles tried to avoid bloodshed if possible and achieve victory through intimidation or treaty.
France includes chapters on the slow and uneven innovations in weapons, armour, and siege engines. Some localities advanced faster than others, and it took a long time for any one technique or improvement to become common practice. He overviewed the changes to castles and fortifications, from dirt mounds with wooden structures to small stone structures, to the huge sprawling city-defending castles with huge protective walls, like the Edwardian Conwy Castle in Wales. Initially kings were the only ones with the wealth to build in stone but eventually the nobles could afford modest stone structures as well. He also has chapters on the development of cavalry, infantry, the confusing use of mercenaries, and the necessary qualities in a good military commander.
In the final chapters he outlines how the European mindset fared and adapted to warfare in the Middle East during the Crusades. Cavalry became more central in the Holy Land, as much land was desert, there was limited water, and much distance to cross between locations. The First Crusade fared so well, in France's estimation, because the troops had had a long walk through Europe and Asia Minor and had actually managed to form a high level of cohesion, as compared to most medieval military expeditions. The huge force was governed by a council of lords, each strong and trusted by their men. The common goal of Jerusalem, mixed with shared suffering along the way, helped forge them into a united group. Granted there were still problems and bickering, but compared to other small scale military endeavors of the time, the fact that they succeeded in reaching their destination and had a successful mission owes much to the cohesion that developed. Some of the European "codes" of warfare were carried over into the Middle East, such as ransoming the aristocracy.
In summary, during the Middle Ages Europe was a land governed by warfare. The landowners fought to defend their holdings and expand their territory and influence. Wealth was measured in land, so the families with the most land were the richest. They could afford the best weapons, horses, weapons, and could hire the most troops to fight for them. Though existing sources are rather sparse France used what was available and provided a good introduction to the topic of medieval warfare.