The rhetorical technique used in this excerpt is shift.
understatement.
logos.
parallelism

Shift

Explanation:

During the first four years of the last war the Allies experienced nothing but disaster and disappointment. That was our constant fear one blow after another, terrible losses, frightful dangers. Everything miscarried. And yet at the end of those four years the morale of the Allies was higher than that of the Germans, who had moved from one aggressive triumph to another, and who stood everywhere triumphant invaders of the lands into which they had broken.

Hope this helps :)