Reserve ships freed first-line forces for the decisive theatre

The guard-ship principle endured because a small standing nucleus could rapidly absorb reservists and other available sailors. By the eve of the First World War, older ships held in reserve were often unsuitable for the primary theatre but still valuable in secondary or tertiary duties. Mobilizing them allowed the navy's best peacetime ships to concentrate in the decisive theatre without abandoning trade protection, presence, and risk management elsewhere. Naval treaty limits later made it harder to retain large pools of older hulls for this purpose.