Leaders of the House Foreign Affairs Committee swept into Beijing last month to meet with Chinese officials, carrying a plea from Washington: if Iran were to be kept from developing nuclear weapons, China would have to throw more diplomatic weight behind the cause.
In fact, the appeal had been largely answered even before the legislators arrived.
In June, China National Petroleum signed a $5 billion deal to develop the South Pars natural gas field in Iran. In July, Iran invited Chinese companies to join a $42.8 billion project to build seven oil refineries and a 1,019-mile trans-Iran pipeline. And in August, almost as the Americans arrived in China, Tehran and Beijing struck another deal, this time for $3 billion, that will pave the way for China to help Iran expand two more oil refineries.
The string of energy deals appalled the Democratic chairman of the Foreign Affairs Committee, Representative Howard L. Berman of California, who called them exactly the wrong message to send to an Iran that seemed determined to flout international nuclear rules.
But some analysts see another message: as the United States issues new calls to punish Iran for secretly expanding its nuclear program, it is not at all clear that Washingtons interests are the same as Beijings.
That will make it doubly difficult, these analysts say, to push meaningful sanctions against Iran through the United Nations Security Council, where China not only holds a veto but has also been one of Irans more reliable defenders.
Their threat perception on this issue is different from ours, said Zalmay Khalilzad, who as the American ambassador to the United Nations under President George W. Bush helped persuade China to approve limited sanctions against Iran. They dont see Iran in the same way as we do.
François Godement, a prominent China scholar and the president of the Paris-based Asia Center, put it more bluntly. Basically, he said, the rise of Iran is not bad news for China.
To be sure, China and the United States, leading members of the club of nuclear nations, share a practical interest in halting the spread of nuclear weapons to volatile areas like the Middle East. And it is in Chinas interest to avoid alienating the United States, its economic and, increasingly, diplomatic partner on matters of global importance.
But beyond that, many experts say, their differences over Iran are not only economic but also ideological and strategic.
The United States has almost no financial ties with Iran, regards its government as a threat to global stability and worries that a rising Tehran would threaten American alliances and energy agreements in the Persian Gulf.
In contrast, Chinas economic links to Tehran are growing rapidly, and Chinas leaders see Iran not as a threat but as a potential ally. Nor would the Chinese be distressed, the reasoning goes, should a nuclear-armed Iran sap American influence in the region and drain the Pentagons resources in more Middle East maneuvering.
Chinese leaders view Iran as a country of great potential power, perhaps already the economic and, maybe, militarily dominant power in that region, said John W. Garver, a professor of international relations at Georgia Tech and the author of China and Iran: Ancient Partners in a Post-Imperial World.
An alliance with Tehran, he said, would be a bulwark against what China suspects is an American plan to maintain global dominance by controlling Middle Eastern energy supplies.
Beyond that, China relies heavily on Irans vast energy reserves perhaps 15 percent of the worlds natural gas deposits and a tenth of its oil to offset its own shortages. The Chinese are estimated to have $120 billion committed to Iranian gas and oil projects, and China has been Irans biggest oil export market for the past five years. In return, Iran has loaded up on imported Chinese machine tools, factory equipment, locomotives and other heavy goods, building China into one of its largest trading partners.
China scholars say that the relationship is anything but one-sided. Iran has skillfully parceled out its oil and gas reserves to Chinese companies, holding exploration and development as a sort of insurance policy to retain Chinese diplomatic backing in the United Nations.
For its part, China has opposed stiff sanctions against Irans nuclear program, acceding mostly to restrictions on trade in nuclear-related materials and orders to freeze the overseas assets of some Iranian companies.
Many experts question how much more punishment Beijing would agree to support. Iran has already been cited three times by the Security Council, with Beijings backing, for flouting prohibitions against its nuclear program.
In each case, Beijing agreed to measures only after stronger American proposals had been watered down and after Russia, the Councils other critic of stiff sanctions and a close ally of Iran, had signed off on the proposal.
One noted Chinese analyst, Shi Yinhong of Peoples University in Beijing, said in a telephone interview this week that China would probably follow much the same course should a new sanctions proposal reach the Security Council.
China will do its utmost to find a balance between Iran and the United States, Mr. Shi said. If Russia joins the other Council members in supporting a new sanctions resolution, he said, China will do its best to try to dilute it, to make it limited, rather than veto it.
But it is unlikely to do so happily. Supporting stronger sanctions might elevate Chinas image as a global diplomatic leader, but the United States, not China, would reap the real benefits.
China is not anxious to jump on this American train, said one Chinese analyst, who spoke on the condition of anonymity in order to freely assess Chinas foreign policy.
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