Protectorate

"A Protectorate, or protected state when referring to a territory subject to this arrangement, is an autonomous territory that is protected diplomatically or militarily against third parties by a stronger state or entity. The protectorate retains formal sovereignty and remains a state under international law, but in exchange for this, they usually accept specified obligations that vary depending on the nature of their relationship."

Source: https://diplomacy.state.gov/encyclopedia/protectorate/