Inter-State Assistance in War Under International Law

Inter-State assistance in war is probably as old a phenomenon as warfare itself and has remained a characteristic feature of today’s international armed conflicts. Russia’s war of aggression against Ukraine shows, however, that crucial legal questions to which such assistance gives rise under today’s multi-layered international legal regulation of war remain understudied. This article, therefore, analyses three main sets of legal implications of inter-State assistance under the jus contra bellum and the jus in bello against the background of the war in Ukraine. First, the article clarifies when inter-State support constitutes an indirect use of force in and of itself, when it would instead constitute assistance to another State’s use of force, and how this distinction matters for assessing the legality of the assistance under the jus contra bellum. Based on an analysis of State practice, the article finds the key delineating criterion to be the proximity of the assistance to the direct use of force by the State that receives support. Secondly, the article analyses how the law of neutrality regulates third States’ assistance to belligerents and argues that this body of law cannot, today, prohibit assistance to the victim of an armed attack. Thirdly, the article explains why it matters to know when States providing support become parties to the inter-State armed conflict themselves and argues that this is the case when their acts have a direct connection to hostilities and occur in close co-ordination with the military operations of the State receiving such support. Overall, the article demonstrates that inter-State assistance in war is today regulated based on a reliable framework, which would, nonetheless, benefit from further consolidation and refinement through State practice.