<p>In August 1918 after years of disappointment, bloodshed and stalemate, Allied troops launched a surprise overwhelming attack on the German Army - a short, four day battle in which the Allied forces advanced 12 miles, more than the total advance of the Somme and Passchendaele offensives combined. Veterans recall how they did it. </p><p>Presented by Dan Snow Produced by Megan Jones for BBC Wales</p>