Trench warfare
British public opinion often repeated the theme that their soldiers were  "lions led by donkeys".
World War I generals are often portrayed as callously persisting in repeated hopeless attacks against trenches. There were failures such as Passchendaele, and Sir Douglas Haig has often been criticised for allowing his battles to continue long after they had lost any purpose other than attrition. Haig's defenders counter that the attrition was necessary in order to cause attrition in the German army.
The problems of trench warfare were recognised, and attempts were made to address them. These included improvements in artillery, infantry tactics, and the development of tanks. By 1918, taking advantage of failing German morale, Allied attacks were generally more successful and suffered fewer casualties; in the Hundred Days Offensive, there was a return to mobile warfare. However, if both sides have comparable equipment, capability, and conditions, the trench is still seen in modern wars.

See also: Harry Patch
Mercy dog
Sangar (fortification)
Underground living

References: Notes

Bibliography

External links: 
Johnson, Patrick In Depth: A century of mud and fire, BBC News, 27 June 2006
Historic films and photos showing "Trench Warfare in World War I" at europeanfilmgateway.eu
World War I Tanks at Tanks Encyclopedia
YouTube video of excavated and restored British WWI trench near Ypres
Preserved Trenches Sanctuary Wood, Ypres, Belgium
Trench Warfare | Apocalypse World War
Trench warfare: Britannica.com article
YouTube video of Hill 60 on the Messines Ridge, site of a huge underground explosion
William Philpott: Warfare 1914–1918, in: 1914-1918-online. International Encyclopedia of the First World War.
Jeffrey LaMonica: Infantry, in: 1914-1918-online. International Encyclopedia of the First World War.
Stéphane Audoin-Rouzeau: Weapons, in: 1914-1918-online. International Encyclopedia of the First World War.
Markus Pöhlmann: Close Combat Weapons, in: 1914-1918-online. International Encyclopedia of the First World War.
Nathan Watanabe: Hand Grenade, in: 1914-1918-online. International Encyclopedia of the First World War.