Why was Germany's attack of France through Belgium a big deal to the British?

Answer 1

The 1839 Treaty of London was an international agreement between several European powers on the one hand, and Belgium on the other. Among the European powers signing the Treaty were Great Britain and Prussia. Germany was a successor state to Prussia; when Germany united into a nation the Prussian kings became the German Emperors, so the Treaty was still binding on Germany. The pertinent article of the 1839 Treaty of London bound all signatories to forever respect and defend the perpetual neutrality of Belgium. So, when the Germans invaded Belgium in 1914 they were not only breaking their solemn promise under the 1839 Treaty to not make war on Belgium, they were ignoring the fact that they had promised to defend Belgium if anyone did attack Belgium. When the British foreign minister reminded the Germans of their obligations under the 1839 Treaty, the German foreign minister scornfully denounced the Treaty as "a scrap of paper". Rather than allow the Germans to cynically do as they pleased as they set about attempting to dominate Europe, the British honored their obligation to defend Belgium under the 1839 Treaty.