We signed an ABM treaty with the Russians, back in the 70s, I believe, which China was not a signatory to. We violated the treaty, at the very least, in spirit, during the Star Wars era, and we continue developing ABM technology today in violation of that treaty, although we have never begun installation of a continental missile defense system.

China is perfectly free, legally, to develop and deploy such a system. And since they are smart enough to realize their dubious friendship with N Korea could end unexpectedly at any moment, I wouldn't be surprised if they haven't already started. In spite of the potential for war with their immediate neighbors as I indicated earlier, Japan is a potential long-term threat, and the US has nukes there, as well as the Phillipines and S Korea, and a Navy capable of delivering theater weapons.

China doesn't need to have a ballistic missile defense system to deter Russia or the US since all they need is a half-dozen or so lucky hits to provide a credible retaliatory response. But if either of those countries develops a viable system, their deterrent collapses.

China does not base her long-term strategies on who her allies and rivals are now, because she knows 100, or 50 or 10 years down the line everything may be different.

I do agree with you on one thing, it would be foolish for us to move unilaterally or unpredictably on this, it would initiate a domino cascade with unpredictable consequences. But I think if we can convince both the Chinese and the Soviets that it is in their best interests, which I'm sure they already realize, something could be made to happen that everyone could live with. At the very least, it gives us a bargaining chip to help persuade China to lean on the Koreans. There are already signs the latter are now more trouble to the PRC than we are, and a nuclear exchange between N Korea and anyone else, even one which does not involve China, would dump a lot of fallout on their territory.