Commentarii de Bello Gallico

[Arjos]

I'll skip the roman one as I think is quite known...
Just few infos about Germanic and Celtic warfare:

The common view for the germanic tribes regarding military actions was centered on the raid, it was conducted by small bands from villages each with its own leader...
They all usually responded to the warband chief, who was also the tribe's chief; the goal of the raid was to acquire loot and gain fame within the tribe and among all tribes...
Also when the tribe moved for an open battle, the women, the elders and children followed...
When the germanic folks engaged in battle they resorted to infantry wedge formations, with the chiefs always up front; they also opted for series of ambushes by day or night in forest areas...
Cavalry was limited, but by this time started to abandon its skirmisher role for a heavy shock troop...

The Celtic world, especially in the mainland, on the other hand was starting to get more and more urbanized: oppida developed once again and people gathered in a single huge settlement, where administrative and economic life took place...
This affected the military bringing more troops in the form of levies, this brought more influence on the spear and less "champions' confrontations"...
But as a tribe gained wealth and a more solid organization, it could muster superior noble forces, better equipped, on foot and mounted...
These troops, even though capable and highly trained, couldn't be replaced within one generation time...

Insular Celts, still had a less urbanized society, which respected, and sometimes worshipped, champions and heroes...
Open warfare was characterized by cattle raids and aristocratic champions, usually moving on chariots, which engaged in smaller forces...
The south of Britain, had by this time developed into a more organized society, similar to the continent's, but it still held those ancient values of heroic warfare...

Mind you all this is my very limited knowledge, I'm by no means an historian...

As an overall these 3 "world" were quite similar and shared a common ancestry, they were regional evolutions of the same values and concepts...
In a way the germanic way represented the primordial step of chiefs caring for the family and by extension the tribe, leading by example; the insular's was the mid step towards urbanization and the continent's one was the very beginning of this last phase...