Britain had guaranteed Belgium neutrality; Germany's invasion of Belgium brought the British Empire into the war.
The German army surrounded the city on three sides, bombarding it throughout much of the war. To counterattack, British, French, and allied forces made costly advances into the German lines in three major battles.
In the first battle, Britain drove the Germans out of the city in the fall of 1914. The next spring the Germans first used poison gas --
chlorine and mustard -- to try to re-take the city in the second battle.
Third Ypres was the horror show; in three months in late summer/early fall 1917, the city was obliterated. And a half million Allied casualties were the cost of gaining a few miles to the East in what is known as the Battle of Passchendaele.
In his Memoirs of 1938, Lloyd George wrote, "Passchendaele was indeed one of the greatest disasters of the war ... No soldier of any intelligence now defends this senseless campaign ..."