Just like in my Romans Politics series, the Roman army did not start off the way we remember it today. When speaking about the Roman legions, most people instantly think about the Roman cohorts. These were very powerful units but the Cohort system was only adopted later...
Phalanx
Before the rise of Rome as a regional power, the Italian peninsula had some very powerful Greek influence. This was because the Greeks colonised part of Italy many years prior and trade in the area still relied on Greek city-states. The Greeks were famous for the Phalanx tactic that they used and this influence led to the creation of the first Roman Phalanx.
A phalanx unit is a large mass of people armed with shields and spears. Unit cohesion was very important in a phalanx as each person would work hard to support everyone else in the unit. Because of this, the phalanx was extremely difficult to stop unless by another Phalanx. In the early rise of Rome, many of the Italian tribes resisted and had to fight to hold onto their land but with unorganised and loose armies meant that the Phalanx tactic was very effective.
Maniple
Although very powerful, the Phalanx was very slow moving and relied on flat terrain in order to use the tactic most efficiently. The ambitions of Rome did not stop with claiming the coastal territories though. Eventually, Roman armies would march on the mountainous strongholds that were left over in Italy. It was impossible to try to displace these tribes with a Phalanx as it was too easily outflanked and made irrelevant.
Roman generals then started using smaller units called Maniples. These smaller units could move a lot faster and could respond by reforming in any direction that they were needed. This was also the start of the checkerboard formation. By spacing the maniples into a checkerboard, this gave a lot of flexibility to the army as they could more easily manoeuvre around obstacles.
Cohorts
Later on, after the Carthaginian conquest, Rome realised that they had to reform their battle formations one more time as the size of a maniple unit became less and less efficient. By this time the enemies that Rome faced could field massive armies with many new tactics that could negate the Roman maniple. This was the start of the Cohort...
A Cohort would be a bigger, better unit that would be self contained and could act independantly from one another. A cohort would be identical to every other cohort allowing for easier replenishment of troops after a battle. Each cohort would also carry all of its own supplies and camping equipment that would lessen the strain on baggage trains. By creating a uniform army structure, Rome was also able to stop a lot of the competition that the different maniples would have amonst one another as each cohort could easily be reformed or shuffled meaning that units would often be split apart without damaging the effectiveness of the Cohort.
This is what the checkerboard formation looked like.
To be continued...
This was just a quick overview of the advancement of the roman battle tactic over time. Tommorrow I woulkd like to go into the unit specifics as this overview does not even begin to touch the armament or makeup of these seperate units