Belgium was officially neutral before the war (like The Netherlands), but of course when there were signs that a war was likely going to happen, the Belgian army was mobilized in 1914, purely for self defence. It wasn't purely so that the Belgian king and government didn't want to go to war, they couldn't, because Belgian neutrality had been one of the conditions from the neigbouring countries to accept the Belgian independence from The Netherlands. This was defined in the Treaty of London from 1839.
According to the same treaty, The United Kingdom promised to secure the Belgian neutrality against foreign aggression. At that time, it was mainly meant to keep France away, because France had been more in favour of a proposal to split Belgium up between France, The Netherlands and Prussia. France had also sent troops to assist Belgium against the Dutch during the war of independence in 1831.
In 1914, this old treaty between Belgium and the UK was used by the UK as the pretext to declare war on Germany when the Germans invaded Belgium to avoid the Maginot line along the French/German border.