Look up De Martens on the Laws of War on Gallica. I didn't include that bit in my translation but I seem to remember there was quite a lot about it.
In 1798 when the Neapolitans entered the Roman states without a formal declaration of war the French protested loudly that this was a breach of the laws of war, though I'm not sure that they were so concerned when they entered Piedmont and other Italian states.
Like most of the the other laws of war, there was no way of enforcing it - you can't cancel the war just because one side is cheating.

regards,

Susan