The BEF's deployment turned Channel control into more than an opening move: the Royal Navy also had to sustain the Army and preserve a route home. The tradition that the Navy does not abandon the Army still depends on material capacity. Ships take years to build and crews decades to develop, so a rescue obligation cannot be improvised after an expedition is already in danger. Force structure and peacetime training are prerequisites for military evacuation rather than abstract naval interests.
Notable Point · 40:51-42:42
Why naval tradition depends on ships and trained crews
Once the Army crossed the Channel, the Navy acquired an enduring duty to support or recover it, but could honor that duty only with ships and crews prepared before a crisis.