ENTRENCHMENTS.--Entrenchments have been employed in the defence from the earliest times. The Roman walls in Britain, the Great Wall of China, the earthworks in the Russian War of 1854-1855, in the American Civil War of 1861-1864, in the Russo-Turkish War of 1878, and the Russo-Japanese War of 1904-1905 are notable examples. But in no war previous to that of 1914-1918 have they played so important a part.
One of the most famous series of entrenchments in previous wars were those constructed in 1810 by Colonel R. Fletcher, of the Royal Engineers, at _Torres Vedras_. These fortifications extended for 50 miles and contained 126 closed works, mounting 247 guns, and behind these lines Wellington amassed stores and reinforcements until the retreat of Masséna enabled him to resume the initiative. In front of these lines everything that could support the French armies had been removed; behind them Wellington's forces were well provided in every respect. On October 10, 1810, Masséna was confronted by the entrenchments, the existence of which had been kept a profound secret, while their strength prevented them from being carried by assault. Before the end of October a Portuguese spy wrote to Wellington: "Heaven forgive me if I wrong the French in believing they have eaten my cat" (Napier). During the night of November 14-15, Masséna broke up his camp and withdrew. But it was not the lines of Torres Vedras which won back the Peninsula. Spain and Portugal were saved by the bold march northwards { 83} to Vittoria. "In six weeks Wellington marched, with 100,000 men, 600 miles, passed six great rivers, gained one decisive battle, invested two fortresses, and drove 120,000 veteran French troops from Spain" (Napier).
DEFENSIVE SYSTEMS.--"Whether it is the intention of the commander to resume the offensive at an early date or whether it is likely that the defensive system will be occupied for a considerable period, the principles on which the construction of all defences should be undertaken are the same. All defensive systems should be planned from the outset in such a way that they can easily be adapted to the requirements of a prolonged defence. The ground must be thoroughly reconnoitred and should at the first be divided into a series of tactical posts and defended localities. These posts should be self-supporting, but should be so sited that the garrisons mutually support each other by fire. The gaps between the posts must be covered by the fire of the garrison of the posts, and machine guns may also be sited to bring fire to bear from positions in rear and to the flanks" ("Infantry Training, 1921"). This principle must govern the choice of the position to be defended as well as the organisation of the position for defence, and troops detailed for the defence of an area must continue to improve the defensive arrangements in that area until such time as the offensive is resumed.