Trench warfare
Trench warfare prevailed on the Western Front from late 1914 until the Germans launched their Spring Offensive on 21 March 1918. After the buildup of forces in 1915, the Western Front became a stalemated struggle between equals, to be decided by attrition. Frontal assaults, and their associated casualties, became inevitable because the continuous trench lines had no open flanks. Casualties of the defenders matched those of the attackers, as vast reserves were expended in costly counter-attacks or exposed to the attacker's massed artillery. There were periods in which rigid trench warfare broke down, such as during the Battle of the Somme, but the lines never moved very far. The war would be won by the side that was able to commit the last reserves to the Western Front.
Cecil Vandepeer Clarke
When production of the limpet mine was under way, Clarke started work on a radical and ambitious project. Drawing on his experiences of trench warfare in the First World War and his particular expertise in tunnelling and explosives, he drew up a proposal for an armoured trench-forming machine. Although the war had started, there was little real fighting on the Western Front and this period became known as the Phoney War. However, Clarke reasoned that eventually Germany's much vaunted "Siegfried Line" must be attacked. His trench-forming machine would drive through the earth advancing 2000 yards in a single night, men and tanks following in its wake in the relative safety of the trench it had formed.