The only thing not violating Belgian neutrality guarantees is it keeps the British from entering the war in 1914. I'm sure that unrestricted submarine warfare would bring them in by 1915 or early 1916.
If the British stay out of the war in 1914 there may not be a war to join in 1916. France is on the ropes and nearly loses Paris in OTL, without the UK there is a strong chance they don't survive the year. Russia is likely to either negotiate in late 1914 after the fall of Paris or surrender by the end of 1915. Look for the German border to encompass the key iron mines at Briey, a general movement of the border 15-20 miles into France, annexation of Luxembourg, and immediate pressure on Belgium and the Netherlands to join the MittleEuropa trade union.