Recognizing the existence of the interrelationship between strategic offensive arms and strategic defensive arms, that this interrelationship will become more important as strategic nuclear arms are reduced, and that current strategic defensive arms do not undermine the viability and effectiveness of the strategic offensive arms of the Parties.
This apparently is the phrase which has many concerned. Explicitly is says nothing. Implicitly it states a strong nexus between offensive and defensive weapons. Having been through these discussions in the 1970s with the CTBT negotiations, one can imagine the intent of each party and the hours and drafts of this statement. Each party believes it contains what they had intended but like so many diplomatic statements its meaning is vague at best.
The other issue is that this is in preamble, not the body of the agreement. Thus for English law advocates it is meaningless yet for Russian readers it sets an unchangeable path forward. How this would be interpreted is an issue and it is an issue for the future. Will there be disputes, indeed, there always are. Is this worth signing, the real issue is China and not Russia. That is the problem.