The alternate history turns on a diplomatic choice rather than a new ship design. Britain and Japan both had reasons to resist folding battlecruisers into the capital-ship category, while the United States had reasons to preserve a simple, restrictive structure. Washington recognized capital ships, aircraft carriers, and an unrestricted residual category, so protecting battlecruisers required writing a fourth category into the agreement. That disagreement changes what each navy may retain, replace, and build in the 1920s.
Chapter · 5:34-10:03
Why battlecruisers need a separate treaty category
The opening problem: the Washington treaty divides ships into capital ships, carriers, and an "other" category, rather than treating battlecruisers as a distinct class.