Joining the treaty would require Stalin to accept limits rather than claim superiority over Britain and the United States. Equality with those two powers would provoke a Japanese demand for equal status. Even Soviet parity with Japan would lead Britain and America to seek more ships against the enlarged threat. French and Italian acceptance of a lower position would also become contentious.
Notable Point · 2:01:37-2:09:50
How Soviet parity could destabilize the London treaty
A Soviet demand for naval parity would trigger matching claims from Japan and pressure Britain and America to expand again.