Why enlarged British light cruisers need not trigger an arms race

Who would respond if Britain built twelve-gun Leanders and nine-gun Arethusas under the treaties?

No navy would have to answer directly; most were constrained by existing programs, resources, or acceptance of Britain's need for many commerce-protection cruisers.

The United States could accept the ships as part of Britain's global trade-defense burden, while Japan, Italy, and Germany were constrained by programs or industrial capacity already in motion. No treaty provision requires another navy to copy a legal British cruiser. Germany might revise Plan Z, but not in a way that solved its underlying resource problem. The deeper consequence is that later Town and Crown Colony designs would begin from a stronger baseline.