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 _rôle_ 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.
THE SECOND BATTLE OF THE MARNE (July 18, 1918).--The great German offensive of March-June, 1918, was renewed on July 15, when the artillery preparation opened shortly after midnight and troops were poured across the Marne in small boats and over pontoon bridges. The attack was not unexpected. Adequate reserves were ready and in place, and a heavy counter-bombardment on the German troops in their positions of assembly, close to their front-line trenches, caused heavy casualties. The Germans succeeded in penetrating the French and American positions in parts of the 50-mile front to a maximum depth of 4 miles south-west of Reims, but on the Plains of Champagne little progress was made and the attack lost its momentum. During the attack of March 21, 1918, the advance was not held up until it was within striking distance of its ultimate objective, and the offensive on the Aisne in May, 1918, secured an advance of 12 miles. Captured documents showed that the attack of July east of Reims was intended to reach the Marne at Eperney and Chalons, an advance of 21 miles. A feature of the earlier days of the battle was a spirited counter-attack near Fossoy (on the extreme left of the German forces) by a division of the American Army which thrust the Germans behind their first line and captured upwards of 1,000 prisoners, the ground regained in the river bend being consolidated and held by the American division. The battle continued for three days before the German { 50} attack was brought to a standstill, and at 4.80 a.m. on July 18 a counter-attack by the French, American, and Italian forces changed the whole aspect of the campaign, and led to the final triumph of the Allies and to the downfall of the Central Powers.