A few quick thoughts in response to this good question. Because of New START, US & Russia mutually monitor the size, status, composition of each other's strategic forces. China knows quite a bit about the other 2's forces as long as they are in compliance w/ the treaty. 1/ https://twitter.com/pranayrvaddi/status/1262745874918064128

We are all so used to a world with strategic arms control that we forget in the early days of the Cold War it took huge effort by both sides just to estimate the size, status, capabilities of the other’s forces. 2/

US still struggles to make these estimates correctly with respect to countries or weapons not covered in such treaties, e.g., North Korean strategic forces, Russian non-strategic forces, etc. It takes a huge national intelligence effort. 3/

In a world w/o New START China has to figure this stuff out on its own and potentially build larger strategic forces (something it has historically been reluctant to do) to guard against mis-estimates. Right now it can be VERY sure of size of US & R forces. 4/

Bottom line, the transparency about US and Russian strategic forces provided by the treaty is not a selective good. It is essentially provided to everyone in the international system—including China, the most likely other nuclear competitor. 5/5