The Roman army was founded around 500 BC and fragmented either 13th or 15th century.
Early Republic.
During the Republlic, the army were conscripts. But in late republic, the army became professional, the army was better equipt. The army was ranked by class.
Late Republic.
They were given a pension and an small bit of land, when they retired, none romans could have joined the army, and given citizenship.
Empire 27 BC-235 AD
The Roman army was at it's largest, 4000 thousand, during Septimius Severus. The army was devided, citerzens Legions (heavy infranty), non citerzens Auxzerly (light infrantry).
Empire 235-610
The size of the army was the same but equitment wasn't, helmets now had a nose guard, no ear holes and scale bronze armour, the army was no longer divided but this created problems as Romans didn't realise how repended on non Romans recriuts.