China's Ambassador to the United Nations Zhang Yesui declared last week that it is "too early" for new sanctions on Iran. As China takes over the rotating presidency of the Security Council, the Obama Administration should consider why, exactly, Beijing is so reluctant to punish Tehran's refusal to make a deal over its nuclear program.

The usual answer is that China has large commercial interests in Iran, particularly in energy, and would be reluctant to impose sanctions that would put them at risk....