German invasion of Belgium The German invasion of Belgium was the first campaign of World War I and the cause of the entry of Belgium and the United Kingdom of Great Britain and Ireland ( plus her colonies ) into the conflict on the side of the Allied powers of World War I . Background . On July 28 , 1914 , following the assassination of Austrian archduke and heir apparent to the throne , Franz Ferdinand , the Kingdom of Austria - Hungary declared war on Serbia . This started a chain reaction of political events : Serbia ' s ally Russia joined the war on Austria , Austria ' s ally Germany joined the war on Russia and Serbia , and Russia ' s ally France declared war on both of the Central Powers . The Austrians and Hungarians were sidelined at the beginning of the conflict , because the Germans were the first to make their move . Their commanders agreed on initiating the Schlieffen Plan , which was a complex invasion route to defeat France and Russia . The plan was to have one force hook through neutral Belgium and enter France from the north , while another German army led a holding battle on Germany ' s borders . Another German army would invade Russia and capture Moscow , ending the war . This was what the Germans followed , invading the neutral and weak country of Belgium . Invasion . Belgium had a weak fighting force of 117 , 000 troops , but the Belgians erected several defensive positions to keep out any invaders . The Germans invaded on August 4 , declaring war on all of the Allied Powers . The 1st Army , 2nd Army , and 3rd Army were sent to invade Belgium , despite its declaration of neutrality in World War I . That same day , an advanced force of six German brigades crossed the German border and attacked the Belgian city of Liège , which was a weak city yet surrounded by a ring of fortresses . The Germans quickly seized the city , but the forts threatened the Germans ' timetable . The Germans bombarded the Belgians with Big Bertha artillery and Krupp gun s , but the city only fell on August 16 . The real German invasion was supposed to occur on August 15 , but the resistance in the city of Liège had sidetracked them . On August 18 , the Germans made their real invasion , and despite the delay , the Germans were able to advance down the Meuse River and capture Namur . On August 20 , the Germans under the command of Alexander von Kluck seized Brussels , forcing King Albert I of Belgium and his government to retreat to the vital port city of Antwerp , which they made their capital . It was pounded away at by German artillery and mortars , but Belgian resistance stiffened as the war turned from fighting an invading force to defending their monarch . The Germans did not subdue the defenses of the city until October 10 , 1914 , when the last elements of Belgian opposition surrendered . The stiffness of Belgian resistance and fear of guerrillas and snipers behind the lines caused the Germans to order the execution of over 670 civilians at Dinant . Further massacres and executions followed , leading to the invasion of Belgium being called the Rape of Belgium . Attempts were made after the war to excuse these barbarties , and even to suggest that they had been invented for propaganda purposes . Aftermath . After the invasion of Belgium , the United Kingdom of Great Britain and Ireland declared war on Germany and Austria - Hungary , honouring the Treaty of London ( 1839 ) . Along with Britain was Japan , South Africa , New Zealand , Australia , Rhodesia , British Raj , Canada , and several other of Britain ' s allies and protectorates . This was the first sign of when the war turned against Germany , as the French troops in the war were later saved by the British on various occasions , such as at the First Battle of the Marne . Germany remained in control of Belgium until their surrender in November 1918 , managing to hold onto Brussels until the end of the war . Belgium was used by the Germans as a spring - board for their invasion of northern France and their advance on the French capital of Paris , which was halted by stiff French and British resistance at the Marne . The Germans had over a million soldiers defending Belgium throughout the conflict , and most were either killed in the campaigns in France , wounded , or surrendered following the surrender of Germany . After the war ended , Albert I was reinstated as monarch and he returned to Brussels to lead his nation once more , ruling until his death in 1934 .