King Leopold of Belgium does what he wants and declares war on the Netherlands with the following outcome, see map.
What effect would this have?
Sorry I don't think Belgium can win that war. Prussia and France would love to get involved. Belgium just gave up their neutrality (by waging an agrressive unprovoked war), a war a large part of the population will not accept, trying to conquer areas that are perfectly happy to be part of the country they are part of. They will not find any support abroad, while there will be various countries that would like to get a part of Belgium, Like France and Prussia.
Also the Belgian army might be stronger than the Dutch army (although I suspect the number of defection will be relatively high), it is hard to cross the rivers that are in their way (Meuse, Waal, Lek, Rhine, IJssel, ask Montgomery about them), I doubt the Belgians will be able to cross the waterline.