Between warships and merchant ships: The legal status, operational functions and targeting rules of auxiliary vessels

Abstract Auxiliary vessels occupy an ambiguous space in the law of naval warfare. They possess neither the belligerent rights of warships nor the civilian immunity of merchant ships, yet they are generally treated as military objectives that may be attacked on sight. Despite their growing operational significance, international law provides no treaty definition of auxiliary vessels, and State practice diverges widely on what counts as an auxiliary and how such vessels should be treated in armed conflict at sea. This article examines the legal status of auxiliary vessels, drawing on treaty law, restatements and contemporary practice. Particular attention is given to the open question of whether auxiliary vessels are military objectives by their nature, as several manuals assume, or instead by their purpose or use. The absence of clear criteria – together with the unsettled issues of exclusive control, crew composition, the limits of support functions, targeting, and self-defence considerations – creates practical and legal risks for both belligerents and neutrals. Greater clarity is needed to distinguish auxiliary vessels from ordinary merchant ships performing auxiliary functions and to articulate the limits of support activities that may lawfully be undertaken by vessels without warship status.