In _Forty Days in 1914_ (CONSTABLE), Major-General Sir F. MAURICE does more than revive our fading recollections of the retreat from Mons and the marvellous recovery on the Marne. A careful study of the German documents relating to VON KLUCK'S dash for Paris has led the author to form a new theory to account for the German defeat. Hitherto we have been asked to believe that VON KLUCK'S fatal change of direction, just when he seemed to have Paris at his mercy, was due to an urgent call for assistance from the CROWN PRINCE. General MAURICE holds, on the contrary, that it was deliberately adopted, at a moment when the CROWN PRINCE'S army was undefeated, in the belief that the French Fifth Army could be enveloped and destroyed, in which event "the whole French line would be rolled up and Paris entered after a victory such as history had never yet recorded." Thus, not for the first time, a too rigid adherence to MOLTKE'S theory of envelopment proved disastrous to the Germans' chances of success. It had first caused them to invade Belgium, and so brought Britain into the War at the very outset; it had next caused VON KLUCK to continue his westward sweep after Mons at a juncture when a vigorous pursuit by his cavalry might have turned the British retreat into a rout; and finally it caused him to execute the notoriously dangerous manoeuvre of changing front before an unbeaten foe, and to give JOFFRE the opportunity for which he had been patiently waiting. The fact was that VON KLUCK did not think the British were unbeaten. He could not conceive that men who had just endured such a harassing experience as the seven days' continuous retreat could possibly be in a condition to turn and fight. Not for the first or last time in the War German psychology was woefully at fault. Whether General MAURICE'S theory is correct or not, it is most attractively set forth, and, thanks to the excellent; maps with which the volume is provided, can be easily followed even by the non-military reader. * * * * *