What mistakes did Britain make during World War One?

(My answer on Quora)

It made several mistakes. The biggest one was getting involved in the First World War to begin with. Many see this as the onset of the rapid decline of Britain as a World Power. The entire war should never have been allowed to go beyond a continental conflict in the first place. 

Other mistakes occurred at Gallipoli (1915–1916) and the Dardanelles fiasco, the overestimation of their artillery barrage efficacy at the onset of the Battle of the Somme (1916) and the significant naval tonnage lost suffered at the Battle of Jutland (1916). Extremely high casualties suffered at the Battle of Passchendaele (1917) for very little territory gained was also not the finest hour in the history of the British military.