Belgium's involvement in World War I began with their refusal to allow German forces to pass through Belgium to reach the French borders in order to invade. This was as part of the Schlieffen Plan, an attempt to invade Paris before allies of France could mobilize. Following this refusal, Germany invaded Belgium and attempted to push through to France anyway.

Satire from the publication Punch illustrating Belgium's refusal to allow Germany to pass through to France

Germany then proceeded to take over 95% of Belgium, with only Ypres still under Belgian command. This occupation caused a mass evacuation, with 200,000 refugees fleeing to Britain, 300,000 refugees fleeing to France, with many of the remaining refugees fleeing to the Netherlands.

Several of the remaining inhabitants suffered "The Rape of Belgium", which occurred during this time. Germany feared that Belgium's "guerilla fighters" would retaliate to their acquisition of Germany. This resulted in several war crimes, including the execution of several innocent women and children. This incident was widely recognised as a war crime.