What factors should be considered for creating a Professional Army in a medieval based society? Basically in a Feudalistic society how would a professional army function? What benefits would this create? What drawbacks? Why would a monarch choose to have a professional army?
Would it be like the Roman Empire where the soldiers also acted as police forces and construction as well? In fantasy (take Lord of the Rings) we see massive professional armies like Gondor and Rohan (I will ignore the Dark Lord for purposes of this).
**EDIT: Perhaps I worded my question poorly. I appreciate the answers thus far but its not really what I'm looking for. I'm not after a history of how our (earth) armies came to rise, but rather what conditions would need to change to make professional armies with 15th-16th century technology (less gunpowder). How could you have Professional Armies and still have Counts, Dukes, Kings, Emperors? Would nobles act as officers? What situations would a commoner choose a life as a soldier/guard? Would their need to be a constant state of war to justify this? What sort of economic impact would this have? What economic power would be required to support this? What if the state/kingdom where in a state of Total War? How would magic fit into this? Perhaps Roman empire structure in the 15th century.
The closest I can think of is the transition period of our history from Feudalism into mercenary armies 1500-1600ish, but rather than Mercenary every kingdom decides they need standing armies. I'm hesitant to provide real world examples but will do so for clarification purposes. Perhaps something like the Roman empire structure in the 15th century. Would Chinese and Japanese society have better examples?
Something more akin to the Holy Roman Empire, but with a clear nobility, merchants and commoner class system. Nobles still own the land and still have peasants like Feudalism and draw taxes from these lands. 'Feudalism' exist in the aspect of a time period(technology wise) and not necessarily the social structure as a direct copy. More of a blend of Feudalism (castles, weapons, Renaissance technology less gunpowder).
In the event of a war rather than summoning his vassals and raising his levies the Duke would just muster/mobilize his professional army and the farmers keep farming while the soldiers do the fighting. In emergencies a conscription of peasantry may occur but the idea is that the soldiers fight.**