This paper examines how Britain, through ‘gunboat diplomacy’ campaigns against so-called Arab pirates, overran the pre-existing Gulf suzerain system and became the predominant power in its waters. By filling a gap in the classical English School ‘international society’ expansion thesis, this article describes how and when political and ideational shifts in the Gulf allowed sovereignty to manifest into its present dynastic form. It argues British imposition of rules, norms, and institutions through a series of nineteenth-century Anglo-Arab treaties against Arab ‘pirates’ broke traditional conditions of divisible sheikhly authority to embed a new telos of sovereign indivisibility, facilitating indirect colonisation. Colonialism as an overlooked primary institution in the classical international society expansion story reinforced political inequality to create dynasticism to simplify colonial statecraft. The 1836 Restrictive Line was a central institution introduced by Britain to manage the transition from divisible to indivisible authority. Drawing from colonial archives, the paper argues that British control over cross-coastal movements through a Restrictive Line reinforced domestic sovereignty of British treaty signatories while weakening agency of maritime sheikhs outside the Anglo-Arab treaties framework. This unsettled traditional structures, transforming maritime tribal confederacies from participation to compliance and reconfiguring Gulf coastal security imperatives for treaty-signatory sheikhs from sea to desert.