Short answer: epic shitstorm.
Long answer: Britain won't stand for it. I am not sure they would declare war, but seems quite likely. Belgium's neutrality was guaranteed by the Powers (Prussia included) under the terms of the Treaty of London, and such a blatant breach of it would make Prussia look very emphatically BAD to everyone.
It would convey the idea that the Germans are up to shake the European balance, not to simply readjust it. The Italian support to France that was under discussion IOTL is more likelier to materialize. Austria might feel much more confident to enter the conflict (Italian guarantees that IOTL were not given were part of the conditions they had set to intervene on the French side) although the Austrians might also keep out anyway because of their internal policy issues on the matter, namely, the Hungarians being unhappy with such an intervention. IF Austria intervenes, however, Russia might cite the unjustified German aggression to eschew her secret treaty obligations to Berlin.
Basically a massive European gangup on Prussia is in the cards, and the Prussians would need a grand-scale repeat of the previous century's "Miracle of Brandeburg" to emerge the conflict with any sort of great power status.
While I don't think that a BEF on the Continent is likely, a British blockade might happen.
Even Denmark might consider settling scores in the right circumstances.
The problem of course is that this premise is unlikely. Even if no Grand European Coaliton to take down Prussia materializes, and the Prussians still somhow manage to win the war with France, Germany would have lost a great deal of diplomatic credibility and international clou, which would make any success untenable lomg-term. I don't see Bimarck not foreseeing that, since securing the diplomatic standing of Prussia and the efficacy of its alliance was a primary concern for him.