Tiny Belgium bore the brunt of the initial German offensive against the allies, as the Germans tried to bypass the strong French frontier defences by invading Belgium and then striking at Paris. The Belgians had a treaty of neutrality with Germany, but the Kaiser famously referred to it as a “scrap of paper” and sent his troops pouring into Belgium.
Despite being hopelessly outmatched, the Belgians put up a defiant resistance which held up the German advance until British and French reinforcements could arrive. The Belgian resistance threw off the German time table for invasion and may have in fact cost Germany the war.
The Germans would never forget the role that the Belgians played in unsettling their war plans, and so treated occupied Belgium extremely harshly. Any resistance was met with executions and other harsh measures. Civilians were conscripted as slave labour to work in German war factories and on farms.
Most of Belgium was occupied by the Germans and their capital city taken, but the Belgians did not surrender and with French and British help held on to a sliver of their country on the western front. It was on the Belgian section of the front that most of the bloodiest fighting took place. Eventually as the German front collapsed in 1918, the allies were able to liberate most of the country and the Belgians were freed.