Is it though? It was civilian infrastructure in international waters, and no formal war has been declared by either side.
I think the problem might be proportionality-- the reasoning would go something like: they could have told the Germans not to let the gas through, and that would have achieved the same effect without violence/destroying civilian infrastructure, so the attack wasn't permitted, or they achieved only a mild military advantage, since the gas could be exported through Azerbaijan, Georgia and Ukraine itself, so it wasn't permitted, and the more I think about it the more I think it actually wasn't.
> Since 1945, developments in international law such as the United Nations Charter, which prohibits both the threat and the use of force in international conflicts, have made declarations of war largely obsolete in international relations
> Declarations of war have been exceedingly rare since the end of World War II.
> In his study Hostilities without Declaration of War (1883), the British scholar John Frederick Maurice showed that between 1700 and 1870 war was declared in only 10 cases, while in another 107 cases war was waged without such declaration
Most of these statements have source references.
While this doesn't cover the whole history, you are welcome to bring up sources contradicting my statement.