The relationship between the two ship types comes from the fleet structure around them. Before the First World War, torpedo-boat destroyers developed while navies remained free to build cruisers in many sizes and numbers. The Washington treaty capped cruiser size and gun calibre, encouraging rivals to exploit the full allowance even though it did not yet cap cruiser numbers. A fleet choosing 6,000-ton cruisers therefore risked meeting much more powerful 10,000-ton opponents rather than gaining a simple numerical advantage. This explains why the destroyer-doctrine answers belong inside a cruiser-design study: both types had to be judged as parts of the same interwar force.
Notable Point · 3:23-4:40
Why treaty cruisers and destroyers became one force-design problem
The Washington treaty's cruiser ceiling changed the relationship between cruisers and destroyers because a navy could no longer assume numbers of smaller cruisers would offset rivals building to the full 10,000-ton allowance.