(ix) _There should be good lines of withdrawal_, and these should be horizontal, or only slightly oblique, to {90} the main position, and not parallel with the general alignment. This is a point of the first importance, for if the Lines of Communication lead straight to the rear a force that is overwhelmed by the attack can withdraw to selected positions and towards its base, if it can keep the line intact and prevent its flanks being turned. A wide base, with alternative lines of approach, is of the greatest value, and when there is undue risk of the Lines of Communication to a base being intercepted, an alternative base, with lines of withdrawal thereto from the unexposed flank, is an acceptable safeguard, as the defence can be protracted while the withdrawing force concentrates upon the changed base. Such a change of base was effected by Marshal French during the _Retreat from Mons_, and amongst many historical examples may be quoted General McClellan's transfer of the _Army of the Potomac_ from the York to the James River in July, 1862, during the _Seven Days' Battle around Richmond_. General Grant changed his base no fewer than five times during the _Campaign in the Wilderness_ (May, 1864), from Washington to Orange and Alexandria Railroad, then to Fredericksburg on the Rappahannock, then to Port Royal, further east on that river, then to White House on the Pamunkey (a branch of the York River), and finally to the James River. "His army was always well supplied, even his enormous numbers of wounded were carried straight away to the base and thence to Washington, without any difficulty, and he had no obstacles whatever to fight against as regards either feeding his army or keeping up the supply of ammunition" (Henderson). In withdrawing a defeated wing it may even be advantageous to rally the troops at a point distant from the field of battle, and to cause the pursuer, uncertain as to the direction of the retreat, to make detachments which can be overthrown by sudden counter-attacks, or to lure a pursuer from the field where their presence is required, as Grouchy was lured after Napoleon's defeat of the Prussians at Ligny {91} (June 16, 1815). The object of Napoleon's attack on the Allies was the separation of Wellington's Anglo-Belgian force from the Prussian Army under Bluecher, and after the defeat of the latter at Ligny the Emperor directed Marshal Grouchy to pursue the Prussians and to drive them eastwards. Grouchy conducted a leisurely pursuit and engaged an insignificant part of the Prussian Army (_The Battle of Wavre_, June 18-19, 1815), while the main body of the Prussians moved westwards and assisted in the overthrow of Napoleon at Waterloo.