Franco-Prussian War
With the defeat of the First Army, Prince Frederick Charles ordered a massed artillery attack against Canrobert's position at St. Privat to prevent the Guards attack from failing too. At 19:00 the 3rd Division of Fransecky's II Corps of the Second Army advanced across Ravine while the XII Corps cleared out the nearby town of Roncourt and with the survivors of the 1st Guards Infantry Division launched a fresh attack against the ruins of St. Privat. At 20:00, the arrival of the Prussian 4th Infantry Division of the II Corps and with the Prussian right flank on Mance Ravine, the line stabilised. By then, the Prussians of the 1st Guards Infantry Division and the XII and II Corps captured St. Privat forcing the decimated French forces to withdraw. With the Prussians exhausted from the fighting, the French were now able to mount a counter-attack. General Bourbaki, however, refused to commit the reserves of the French Old Guard to the battle because, by that time, he considered the overall situation a 'defeat'. By 22:00, firing largely died down across the battlefield for the night. The next morning, the French Army of the Rhine, rather than resume the battle with an attack of its own against the battle-weary German armies, retreated to Metz where they were besieged and forced to surrender two months later.

Q: Which army's defeat compelled Prince Charles to attack?
defeat of the First Army,