How to make an agent stop accompanying an arthe

Only way to "split" an army seems to be having two generals. When you order a general to march to another general, you get a possibility to change troops from on army to another.

Compared to other TW games this seems like a huge drawback to be, because you dont seem to be able to recruit troops to undefended city if there is no general present nor can you leave troops to defend a city when your army marches on. City garrisons are usually not enough to defend a city and you need to carefully place your armies.

It does have a certain point, because roman armies were strictly hierarchical and there had to be a general leading it. It leads some inconsistencies tho, like in case your general is killed, you will be able to recruit a new one to that spot immediately.

As mentioned by @5pike, the red skull symbolises attrition.

There are several reasons why an army or a fleet can suffer from attrition, here are some:

• Your army is currently laying siege (attrition applies after you end your turn)
• Your army is besieged by an enemy (attrition applies when your turn starts)
• Your army is currently suffering due to certain events (plague, sickness, earthquake, etc.)
• Your food surplus is negative and your army is currently in a province that imports food.
• Your army is currently in terrain it is not used to (e.g. desert, mountains) and not on a road.
• You cannot pay for your army's or fleet's upkeep (happens when a player has no settlements)
• Your fleet is currently in deep water