This. For the British in both WW1 & 2, the expansion of their army took place during hostilities, not prior. This resulted in a massive influx of untrained manpower that had to be trained and equipped. This took time and diverted trained and experienced troops from use at the front, which was a good thing. The British also decided that their newly raised troops should, ideally, meet the same standards in tactical training, regardless of unit. No highly trained, highly trained 1st line troops backed up by more static, under-trained 'fortress' troops, which the continental armies so often utilized.Some make too much of the losses to the pre war regular army. Indeed there were significant losses but also there was a steady drip of troops moved to training the New Army and promoted to newer and Territorial units. My grandfather was a pre war Regular who served all through the war as did several of his contemporaries. They disappeared from sight but were still there passing on their skills. It was not until 1917 that the army could generate enough of it’s own experienced soldiers to cease to lean on pre war Regulars. In 1915 those Regulars were still a key component in the system and in 1916 new self generating trainers were starting to come in.