On a scale before unknown in Western Europe, and save for the coincident operations in the Eastern theatre of war, unexampled in history, the succession of events named the "Battle of the Rivers" presents illustrations of strategy and tactics of absorbing interest. Apart even from the spectacular aspects of this lurid and grandiose drama, full as it is of strange and daring episodes, the problems it affords in the science of war must appeal to every intelligent mind. An endeavour is here made to state these problems in outline. In the light they throw, events and episodes, which might otherwise appear confused, will be found to fit into a clear sequence of causes and consequences. The events and episodes themselves gain in grandeur as their import and relationship are unfolded. Since the story of the retreat from Mons has been told in another volume of this series, it is only in the following pages dealt with so far as its military bearings elucidate succeeding phases of the campaign.