WW1: Britain, Germany, and Belgium | AskHistorians Archive Observer

It wasn't a 'secret defensive pact with Belgium', it was the Treaty of London, signed in 1839, which stated that Belgium was to defend it's neurtralit against any aggressor, and in return Britain, Prussia (Germany), Austria (-Hungary), Netherlands, France and Russia would step in as guarantors of Belgian sovereignty. The Entente with France was in theory tenuous, but military commitments such as that calling on the Royal Navy to protect the Channel were France attacked made it difficult for the British to step back so easily.

The only way a scenario in which Belgium was not invaded would work would be for Germany to adopt a 'Russia first' policy, and this was ruled out in 1913, when Moltke the Younger, the Chief of the Greater German General Staff, ceased updating the mobilization plans against Russia, committing Germany to a two front war, in which France would be the target.

The only way to attack France was A) to take the route of 1870 through the Franco-German Frontier or, B) Invade Belgium and Luxembourg. Option A wasn't really an option at all, given the rough terrain of southeastern France (the Vosges Mountains, the Argonne Forest), and the French would likely concentrate the bulk of their army their, supported by a line of fortifications just behind the frontier, largely following the Meuse and Moselle rivers south from Verdun.

The only 'feasible' route was through Belgium, and that meant confronting the Belgian army and it's fortifications, and certainly entailing British intervention. An attack against France would have threatened the Channel Ports, and Belgium, while an attack on both had the effect that it did in August 1914: forcing Britain's hand, and drawing the largest empire in history, the world's greatest naval power and sole superpower, into the fray.