If I may shed some light on the question of how long a treaty is valid. Might be important for yhis question but also for others.
According to treaty law, a treaty (or convention) stays valid as long as there is nothing else replacing it or annuling it.
Modern treaties are signed for a short duration and cease to be valid at a certain date, given by the treaty. If not specified, there are three possibilities for a party to a treaty to stop its application:
- a party to the treaty can denunciate it (regulations for this are often given by the treaty)
- If the treay is violated by one of the parties, it can be suspended by another party (like a breach of contract)
- Fundamental change of circumstances, meaning the appearance of circumstances that did not exist when signing the contract (VERY rare)
Other things that can happen to a treaty is that an international armed conflict emerges. During the conflict, treaties can de suspended, but not necessarily suppressed. Last but not least, the treaty may become invalid by the emergence of rule of ius cogens
(an imperative rule of law) on the matter.
Concerning the point that Prussia signed and not Germany, I have to point out to the principles of State succession: When the german empire was created in 1871, it's executive power remained withe the king of Prussia as Kaiser, which could mean (that's an international public law theory) that the German empire is the successor of Prussia and thus takes over all of Prussia's obligations with regard to international law. In which case, Germany was bound by the 1839 treaty when invading Belgium (even if Bismarck called it "un chiffon")
Concerning the 1839 treaty itself, there are in fact 2: the first one, comprising 24 articles, defining the borders of Belgium and stating, in article 7, its neutrality. This treaty has been ratified by Belgium and the Netherlands. At the same time, another treaty, containing only 4 articles, was ratified by France, Austria, Russia, Prussia and Great Britain. it reads: [ces articles] sont considérés comme ayant la même force et valeur que s'ils étaient insérés textuellement dans le présent acte ; et qu'ils se trouvent ainsi placés sous la garantie de leurs dites Majestés.
Meaning that the first treaty has on these staes the same valour as for Belgium and the Netherlands.
So to sum up: the 1839 treaty was still valid in 1914, following the basic principle of treaty law: Pacta sunt servanda.
Germany, as a successor to Prussia, violated this treaty by invading Belgium. Simple fact of international law.
Hope I did not bore you... occasion was just too beautiful to get back to my books ...
AUST, A. Modern treaty Law and practice, Cambridge university press, 2002
THOMAS, BRULS & MINGOU: Droit International Public, P