Since the invasion of Ukraine, Western states have faced a dual red line, i.e., at what point neutrality loss and co-belligerency occur. In this Article, I argue that neutrality law is still relevant regarding arms transfers and troop training, which are prohibited, but is often ill-suited to regulate activities like intelligence sharing, which is not subject to a specific treaty or customary rule. As this Article argues, this is because “impartiality” cannot be described as an abstract and autonomous rule, but rather as an umbrella principle underpinning the law of neutrality. It means that helping a belligerent is not ipso facto prohibited. I also consider that private actors like arms brokers or telecommunication service providers are subject to insufficient constraint, and that qualified neutrality is not yet part of positive law, which is problematic in the event of an aggression. Indeed, if necessary, the aggrieved belligerent remains entitled to use proportionate use of force to end neutrality breach. Then, this Article finds that analogies with the rules governing non-international armed conflicts and responsibility are still required to help clarify belligerency law, even where an international armed conflict is at stake. Drawing on the Nicaragua and the Tadić cases, it seems that “arming and training” constitute use of force, and that “organizing, coordinating or planning” military actions result in conflict participation. Mere “financing, training and equipping or providing operational support” are not enough to become a conflict party, however.
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