The applicability of neutrality law is a contested area of modern international law. Because it determines the options third states are facing in an international armed conflict, the Russo-Ukraine war lends it new urgency. This ePaper analyzes these options from a conceptual and practical perspective. It will, first, theoretically discuss the applicability of neutrality law, rejecting the theories of compulsorily applicable, qualified, and obsolete neutrality based on their conceptual flaws and inadequate incorporation of the emergence of a third option for third states. It embraces optional neutrality based on the finding that it has always been optional. The second part examines the changing customary neutrality law and the related legal uncertainty for states. It finds that this uncertainty reproduces itself and is reflected in the German and Austrian debates and their opinio juris. To encounter this legal uncertainty, the third part details the options available for third states – neutrality, non-belligerency, co-belligerency, and their illegal counterparts.We extend our heartfelt thanks to the Vahabzadeh Foundation for financially supporting the publication of best works by young researchers of the Graduate Institute, giving a priority to those who have been awarded academic prizes for their master’s dissertations.