I've been binge watching Sharpe recently and I started to think that the representation of military tactics of different nations seems similar if not identical. The same in other historical fiction like Captain Alatriste and from what I remember of the Hornblower series, and even fantasy like Game of Thrones.
Now I appreciate that these are all fiction but still it got me wondering whether a) they're even remotely accurate b) if they are how did the 'rules' of war develop over time and c) did different forces have 'specialities' and different ways of approaching battles.
In my mind I'm imagining that maybe they evolved from a common root, the Romans maybe.
Well, Romans certainly laid a common foundation, Vegetius' De Re Military was translated into many languages and well known in medieval Europe.
What dictates your tactics however, is who you fight, and who you have to fight for you. You can especially see this once you look into eastern Europe, Russia and Hungary had different army compositions and tactical approaches to warfare than most western countries.
As an example, you could expect a Hungarian army of 1200's to have a lot, and I mean a lot of light cavalry. not to the extent of Mongols, but some sources mention a third of the army being light horse. Some of these would be nomadic horse archers, but a lot of these were poorer nobles (some nobles even preferred to fight in tourney melees as relatively light cavalry). In battle of Marchfeld (1278), Hungarian light horse kept harassing left wing of the enemy force (Bohemians led by Premysl Ottakar II) for several hours (from dawn to noon) as an organized tactical effort, withdrawing when they were challenged by enemy heavy cavalry.
Another example if battle of Grunwald, where Polish-Lithuanian army used their light cavalry do basically kite away enemy heavy cavalry and win the infantry battle while they were busy chasing enemies they couldn't possibly catch up to.