As ASB as it sounds it could possibly have become a world wide conflict leading to the ruin of several of the European powers
If France or Germany makes a stupid mistake and invade Belgium, yeah, I could see ruin on either country, and the war escalating to three continents.
If France invades Belgium, Great Britain joins the side of Germany. Big time fighting between Russian Central Asia and British India, and between Russian Far East and Japan, who was allied with Great Britain OTL throughout the twentieth century because of the Anglo-Japanese Alliance. Also, huge fighting in Africa between France and the alliance of Great Britain, Portugal, Germany, and Belgium.
If Germany invades Belgium, Great Britain joins the side of France. Less war in Asia and Africa, with Japan taking over some minor islands claimed by Germany, and Great Britain joining France to take over German colonies in Africa.*
This is because in a major European war, Belgium will most likely remain neutral and allied with Great Britain, and by extension, Great Britain will remain neutral. An violation of Belgian neutrality will cause Great Britain to join the war. And a major European war is actually quite likely if Russia agreed to intervene, putting the alliances into play.
*Though there was the Schlieffen Plan created in 1905, asking German troops to invade into Belgium, it wasn't taken seriously by the German military and soon was tossed out.