Collective Security through Alliance Security: Understanding “Regionalised” Collective Security in International Law through the UK’s post-Brexit Alliance Policy

Reeder, Danielle ORCID: 0000-0002-0099-0441 (2025) Collective Security through Alliance Security: Understanding “Regionalised” Collective Security in International Law through the UK’s post-Brexit Alliance Policy PhD thesis, University of Liverpool.

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Abstract

When the United Kingdom (UK) left the European Union (EU) and the security and defence configurations under the EU’s Common Security and Defence Policy (CSDP), the UK withdrew from the EU Treaties without any replacement arrangement for external security. The process of Brexit ultimately revealed three major policy themes: (1) That the international security landscape was becoming more regionalised; (2) The blurred distinction between internal and external security called for a more expansive understanding of collective security; and that (3) NATO had a preeminent role in UK security formulation alongside, above, and in place of the EU’s CSDP. Although the UK government originally proposed establishing a robust, new security and defence arrangement that would include both internal and external security considerations between the UK, the EU, and Europe more broadly, the UK ultimately decided that any anticipated gaps in security and defence arrangements would be sufficiently managed through a network of bilateral and multilateral alliance configurations. The security and defence policy of Brexit pivoted, in particular, to a renewed understanding of its membership in the North Atlantic Treaty Organisation (NATO), seeking to achieve collective security goals through NATO. This result of the Brexit process not only outlined the ‘regionalised’ collective security course for the UK outside of the EU, but also implicitly endorsed an ‘alliance-based’ understanding of collective security. Collective security, as a juridical term under international law, is a system designed to maintain peace and security for the international community by preventing, sanctioning, and responding to threats against, or common to, all States. Premised on the prohibition of the unilateral use of force under Article 2(4) of the Charter of the United Nations (UN Charter), collective security is conventionally associated with the collective enforcement articles within Chapter VII of the Charter; under the consultation and accountability structure of the UN General Assembly and the UN Security Council. Military alliances such as the North Atlantic Treaty Organisation (NATO) find legal basis in the provisions of collective self-defence as enshrined in Article 51 of the UN Charter. Although Article 51 falls within Chapter VII of the UN Charter, any organisation established on the basis of this article is premised on preparing for, and responding to an armed attack against an external threat. Alliances are selective and exclusionary by nature, and present a conceptual contradiction to the universalist ambitions of the UN Charter collective security system. And yet, the legal casings of collective security under international law seem to oppose, and simultaneously necessitate the inclusion of military alliances. This represents an essential conceptual tension at the heart of collective security law. This conceptual tension between the aims of collective security and the function of military alliances is tested by the UK’s policy that centralises and elevates NATO to obtaining the status of collective security. Despite this tension, however, legal justifications that support alliance security as collective security can be effectively made, despite the core inadequacy of military alliances to support the collective security ideal. In other words, alliance security can be ‘apologised’ for. Through the lens of Brexit and the UK’s ad hoc ‘alliance policy’, this thesis outlines the modalities through which to understand alliance security as collective security, whilst also addressing the limitations of those modalities. The policy themes of Brexit correlate to three unique modalities, or ‘apologies’, for understanding alliance security as collective security. This work has labelled and framed these three thematic apologies for alliance security as ‘regional accommodation’, ‘authorised champions’, and ‘disclarity’. These apologies are not alternative views. Instead, they reflect an interworking and overlapping set of legal analyses that fit the square peg of military alliances in the round hole of the collective security doctrine under international law. The aim of this analytical exercise is to highlight, problematise, and unpack the conceptual tension between collective security theory and practice, by examining the State practice of the UK’s ad hoc ‘alliance policy’.

Item Type:	Thesis (PhD)
Uncontrolled Keywords:	alliance security, Brexit policy, collective security, collective self-defence, Common Security and Defence Policy, Global Britain, North Atlantic Treaty Organisation, regional arrangements, regionalised collective security, regionalism, vicarious defence
Divisions:	Faculty of Humanities & Social Sciences Faculty of Humanities & Social Sciences > School of Law and Social Justice
Depositing User:	Symplectic Admin
Date Deposited:	19 Aug 2025 09:44
Last Modified:	19 Aug 2025 09:46
DOI:	10.17638/03191148
Supervisors:	Farrell, Michelle Reynolds, Stephanie Murphy, Ben
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3191148
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