Shortly after midnight on August 4, the German attack on Liege began. It was quickly repelled under heavy fire. Another attack followed shortly and was again turned back. Ten days would pass before the forts of Liege were in German hands.
Informed of the attack, the English issued an ultimatum to Germany to cease hostilities or face war with Britain. No answer was received by 11 pm, August 4th, and so a formal declaration of war was issued by Prime Minister Herbert Henry Asquith.
Sometime in the afternoon of August 4, Siegfried Sassoon enlisted as a trooper in the Sussex Yeomanry (he later took a commission in the Royal Welch Fusiliers). He would, of course, survive the war.
His wartime journals have been recently made available online here.
all their hope & merriment snuffed out for ever, and their voices fading on the winds of thought, from memory to memory, from hour to hour, until they are no more to be recalled.
— Siegfried Sassoon, journal entry, 13 July 1916 (MS Add.9852/1/7, folio 27v)
Drawing by Sassoon.