( CNN ) -- Twelve cannons were fired in Liege , Belgium , on Monday to conclude a ceremony marking the outbreak of World War I 100 years ago . Britain declared war on Germany after it invaded neutral Belgium on August 4 , 1914 . The declaration escalated a conflict -- sparked by the assassination of the Archduke of Austria-Hungary -- into a global war . About 16 million people were killed before the war ended four years later . Belgium was a key WWI battleground . Dignitaries including King Philippe of Belgium , the French and German Presidents and Britain 's Prince William , Duke of Cambridge , addressed those gathered in remembrance Monday . Prince William said the people of Belgium had been among the war 's first victims , and their `` resistance was as gallant as their suffering was great . '' Belgium 's sacrifice and contribution had been pivotal to eventual victory , the prince said . `` Belgium 's resistance in 1914 allowed the Allies to regroup and draw up the battle lines which became the infamous trenches . These trenches have left an indelible scar on your land ; they speak of the horrors of war but also of your forebears ' courage , '' he said . Prince William said recent events in Ukraine showed that `` instability continues to stalk our continent . '' But he said the fact that war was now `` unthinkable '' between the former World War I enemies represented at the ceremony `` bears testimony to the power of reconciliation . '' `` We were enemies more than once in the last century , and today we are friends and allies . We salute those who died to give us our freedom . We will remember them . '' The king of Belgium laid a wreath to commemorate the dead before a bugler sounded `` The Last Post . '' A young girl representing future remembrance then released a white balloon as a sign of peace and reconciliation before the cannon shots rang out . Britain is also holding events across the United Kingdom and in the Belgian city of Mons , where the first British soldier died during the conflict . British Prime Minister David Cameron said in a statement , `` A hundred years ago today Britain entered the First World War and we are marking that centenary to honor those who served , to remember those who died , and to ensure that the lessons learnt live with us forever . ''
Held in Belgian city of Brussels on anniversary of outbreak of World War I. Germany invaded neutral Belgium on August 4 , 1914 , and Britain declared war . Over the next four years , 16 million people died in the conflict , many on Belgian battlefields . Prince William said war was now ` ` unthinkable '' between nations who had fought .