At the _Battle of Waterloo_ (June 18, 1815), the decisive counter-stroke was delivered, in accordance with Wellington's pre-arranged plan, by a force coming from a distance to the scene of action. On the morning of June 17, when Wellington resolved to make a stand at Waterloo, he was aware that the Prussians, who were mostly young troops, had been beaten at Ligny; that Napoleon had, before that battle, over 120,000 men, and that he himself had, all told, 68,000, of whom 31,000, including the King's German Legion, were {48} British. Yet he withdrew from Quatre Bras with the full determination of standing at Waterloo and of fighting Napoleon's army, if Marshal Bluecher would come to his assistance with one Army Corps. Napoleon attacked on June 18 with 72,000 men and 246 guns, against Wellington's 68,000 men with 156 guns, at 11 a.m., but he was unable to shift the line or break through the squares. At 4.30 p.m. one of Bluecher's corps was delivering the promised counter-attack against Napoleon's line of communications. Soon after 9 p.m. Wellington and Bluecher met at La Belle Alliance, Napoleon's headquarters before the battle, and the pursuit was in full swing. Opportunities for restoring the battle and for turning impending defeat into a crushing victory are frequently offered during an engagement. General Lee's thin lines at _Antietam_ or _Sharpsburg_ (September 17, 1862), slowly fed by men jaded by heavy marching, were sorely pressed, but there was a lull in the Federal attack when Hooker's advance was checked. Had General McClellan at that moment thrown in "his last man and his last horse" in a vigorous reinforcing attack, _Antietam_ would not have been a drawn battle, and Lee would not have retired at his leisure into Virginia. Lee's great victory at _Chancellorsville_ (May 2-3, 1863), although marred by the accident which deprived him of Stonewall Jackson, was a striking instance of the success of the Defensive-Offensive system at the hands of a great commander, who defeated 90,000 troops with less than half that number, by a containing defence with 13,000 men and a decisive counter-stroke with the remainder. But while this combined system is regarded by most authorities as the best, when circumstances warrant its adoption, it is the highest test of generalship to seize the right moment to pass from the guard to the thrust. This is the problem which confronted Marshal Foch, the generalissimo of the Allied Forces, during the great {49} German offensive movement on the Western Front in 1918. The defensive _role_ endured from March 21 until July 17, 1918, and although many local counter-attacks were made along the whole battle front, the Allies did not pass from the guard to the thrust until the decisive counter-stroke was commenced in the _Second Battle of the Marne_ (July 18, 1918) on a front of 27 miles from Fontenoy to Belleau, which drove the Germans back across the Marne on July 20.