From the Hague Convention of 1907 regarding ‘the Laws and Customs of War on Land’:
Article 25 The attack or bombardment, by whatever means, of towns, villages, dwellings, or buildings which are undefended is prohibited.
Article 25 The attack or bombardment, by whatever means, of towns, villages, dwellings, or buildings which are undefended is prohibited.
Article 2 of the Hague Convention No. IX of 1907 on naval bombardments in time of war allowed naval forces to fire on certain objects even when the locality concerned was not defended.
[snip]
Generally speaking, it was possible to observe a steady shift, for bombardment to be permissible under inter- national law, from the requirement that a locality be defended to that of the presence of a military objective. 5 By the time the First World War ended, therefore, the law of air warfare had virtually lost its sole codified basis: the prohibition on dropping explosives from aircraft had become invalid and Article 25's field of application had turned out to be extremely narrow. It had been replaced by the very imprecise prin- ciple of the "military objective"
If you continue reading that document it becomes clear there were multiple attempts the codify the rules (eg the Washington Conference talks, the Hague Commission of Jurists) none of which were ratified.
However, the 1922/23 "Hague Rules of Air Warfare" (which were not ratified) are notable in that the discard the idea that defense makes any difference and switched entirely to the military objectives view, with carve-outs to try to protect civilians and monuments.
https://international-review.icrc.org/sites/default/files/S0...
In 1946 the Nüremberg International Military Tribunal stated with regard to the Hague Convention on land warfare of 1907: "The rules of land warfare expressed in the Convention undoubtedly represented an advance over existing International Law at the time of their adoption ... but by 1939 these rules ... were recognized by all civilized nations and were regarded as being declaratory of the laws and customs of war" (reprinted in AJIL, Vol. 41, 1947, pp. 248-249). The International Military Tribunal for the Far East expressed, in 1948, an identical view.
Yes.
It's worth noting that the idea of "international law" really only became a thing with the Nuremberg trials. Before that it was assumed people would do the right thing, or something...