The United States certainly does over-emphasize Iran relative to its own interests, but it does not seem plausible that China's position on Iran would by itself cause what now seems like a trend of US provocations of China, the visit of the Dalai Lama, the arms sales to Taiwan and the public complaints about China's currency policy.
I've doubted the narrative that Russia and China effectively restrain US attempts to harm Iran for some time now. The United States could stop sea-based Iran gasoline imports at any time, for example. The US Treasury Department could also blacklist Iran's central bank today if it chooses. Because of that, I do not see, beyond play-acting, any cause for US frustration at China over China's supposed reluctance to impose new sanctions on Iran.
On the other hand, the United States was not as publicly antagonistic towards Iran six months ago as it is today, and while Taiwan, Tibet and the renminbi have been issues for a long time, the US has usually not as publicly and bluntly challenged China as it is doing today.
I don't know what's going on. Maybe the Obama administration is trying to demonstrate that it is willing to apply maximum pressure in all areas of its relationship with China in hopes of making China more cooperative with sanctions. Maybe China has decided not to cooperate with sanctions, so the United States is expressing its anger and frustration at China. Maybe US/China tension is increasing for some other reason.
What we do know is that no sanctions regime, China supported or not, will cause Iran to give up its nuclear program. US policymakers seem to understand this. The acknowledged effect of sanctions is more to punish Iran than to deter it.
We also know that one effect of sanctions is that they allow the US to postpone its conscious or deliberate acceptance of Iran's nuclear program - which is something US policy-makers, indirectly because of a fear of being perceived as anti-Semitic, have a strong psychological impulse to delay for as long as possible.
It is likely that another effect of sanctions is that Iran would retaliate in ways that are harmful to the two most important foreign policy concerns for the US today, Iraq and Afghanistan and that now is a very sensitive time in both cases. US policymakers should be aware of this consequence of sanctions, but I have not seen much discussion about it. I guess it is possible that the US foreign policy community is overlooking the damage sanctions would do to US regional efforts, but I consider that unlikely.
Most likely, the United States is ambivalent about sanctions in total, and from that position, frustration over Chinese cooperation over sanctions could not be a driving force in the apparent deterioration of US/Chinese relations we see today.
What is causing the deterioration, of which China's new public opposition to the supposed US drive for Iran sanctions is most likely just another symptom rather than a cause? The United States is generally a less powerful country than it was when the careers and perceptions of the US foreign policy community were developing. I guess adjustments have to occur at some time or other, and maybe now is a time that we are seeing the bumpiness of and resistance to the process by which the United States loses its global pre-eminence.
