General. Well, protectorates don't join your wars. Naturally this means you can't set wargoals for them in the war. If you want to give them planets, first take them for yourself and then trade them to your protectorate afterwards.

How do protectorates work in Empire Total war?

Protectorates cede some of their income to their protector nations, in exchange for a pledge from the protector faction to ally itself with it and declare war on enemies invading the protectorate and vice versa. Protectorates also automatically grant indefinite military access to their protector faction.

What does become protectorate?

A protectorate is a state that is controlled and protected by another sovereign state. It is a dependent territory that has been granted local autonomy over most internal affairs while still recognizing the suzerainty of a more powerful sovereign state without being its direct possession.