AI can invade me without declaring war?

[Tanaka]

But if I attack them in my territory I have to declare war? Huh?

[eddieballgame]

Until one's boundaries are determined via a treaty, etc., it is a 'free for all' per grabbing space. Attacking another player's/AI unit is another matter, & requires being at or declaring war.
If a minor, or major for that matter, offers a peaceful resolution to the separating boundaries; take what you can & then agree to the peace offering...if you want to, that is.

[Tanaka]

Until one's boundaries are determined via a treaty, etc., it is a 'free for all' per grabbing space. Attacking another player's/AI unit is another matter, & requires being at or declaring war.
If a minor, or major for that matter, offers a peaceful resolution to the separating boundaries; take what you can & then agree to the peace offering...if you want to, that is.

Thank you for clarifying. I must have clicked on a hidden unit in the minor's territory which did not allow me to enter without declaring war thus the confusion...