Soviet attendance at the 1930 London conference would most probably break the treaty system. The demands a Soviet delegation would bring could be intolerable to several existing treaty powers and collapse the system outright. The counterfactual therefore searches for a narrow route by which the participants might still sign, while treating that outcome as unlikely. Every later fleet estimate depends on the participants finding that compromise.
Notable Point · 0:25-1:40
Why treaty collapse is the baseline
Soviet participation makes a complete breakdown the most likely result; a surviving agreement requires a narrow compromise between incompatible naval demands.