I am attacking a capital while it is settled as a newly joined player

In Ancient World, when I was attacking Sparta in turn 2, if a newly joined player sets Sparta as capital in turn 2, then where do my army go?
Moreover, this player appears as my ally when he suddenly joins in this turn. Then can I directly take him down after declaring war to him in the same turn? Or get my army back if I can't attack him in this turn?
Please help me, I am so confused about the operation.

Latejoiners are called when they join after the game has started and if u attack for example sparta at turn 1 when it is neutral and a player joins the next turn your units will still attack him but instead of militia sparta will get infantry and a gen if he is premium and after a player latejoins he is automatically peaced with all other players but the very next turn automatically all those peace treaties are ended unless you or he sends you or someone else and ally request
this is to prevent other players insta killing a player that latejoins btw