The stated US aim when indirect US-Iranian talks resume this month is to see if the two can revive a 2015 nuclear deal, but Washington's unspoken goal may be to win support from China and Russia to pressure Iran if the talks fail, diplomats said.
Western diplomats have said time is running low to resurrect the pact, which then-US President Donald Trump abandoned in 2018, earning Iran's fury and the dismay of the remaining major powers in the pact - Britain, China, France, Germany and Russia.
Under the agreement, Iran limited its nuclear program in exchange for relief from US, EU and UN economic sanctions. Trump's 2018 decision to reimpose harsh US sanctions prompted Tehran to begin breaching the deal's nuclear restrictions.
The three European nations have worked hand in glove with the United States to try to restore the deal and Russia has been supportive, notably at the Vienna-based UN nuclear watchdog that monitors Iran's eroding compliance.
China, however, has undercut US leverage by buying Iranian oil, throwing Tehran an economic lifeline in violation of US sanctions.
One way to pressure Iran to rejoin the original pact or, if that is not possible, to accept another arrangement would be to persuade Beijing and Moscow that Tehran, not Washington, is the obstacle, diplomats said.
"They need China and Russia," said a Western diplomat on condition of anonymity.
Shifting the onus
While Washington initially got blamed for Trump's withdrawal and reimposition of sanctions on Iran, the onus may gradually be shifting to Tehran as US President Joe Biden seeks to restore the pact.
US and Iranian officials held six rounds of indirect talks in Vienna between April and June. A new round begins on Nov. 29 after a five-month hiatus triggered by the election of a new Iranian president, Ebrahim Raisi, a hardline cleric who said this month that Iran would not back down in nuclear talks.
US and European diplomats are frustrated by what they view as Iran's unrealistic demands, including what one described as a recent insistence that all US and EU sanctions imposed since 2017 be dropped, and their expectations for the talks are low.
This month's talks will be a chance to gauge whether Iran wants to revive the 2015 Joint Comprehensive Plan of Action (JCPOA) and, if not, for Washington to try to garner greater support from China and, to a lesser degree, Russia to put pressure on Tehran.
"(The) Russians are a more helpful partner than China," said a second Western diplomat on condition of anonymity. The best way to get China's support was via diplomacy rather sanctioning Chinese companies that buy Iran's oil, the envoy added.
"We have to talk to China and get them on side," this diplomat said.
Biden and Chinese President Xi Jinping discussed how to "align" their positions before the Vienna talks during their virtual summit this week.
'Good old college try'
The major powers believe the greater their cohesion, the better the chances of bringing Iran back into the deal, said a third Western diplomat.
"It becomes more important when we come to a crucial point in the discussions," the diplomat added, referring to the next round and Western hopes that China might eventually trim its oil purchases.
Brookings Institution analyst Robert Einhorn suggested Washington may wish to show it had gone the extra mile to revive the agreement.
"(The idea is to) make a good old college try ... but if that doesn't work, make sure that the world sees Iran as the reason for failure," Einhorn said.
Shifting the onus to Tehran could prove helpful even if Washington concludes the original deal is dead but tries to fashion another agreement.
The State Department has said it will pursue diplomacy even if the JCPOA cannot be saved.
"We want to present Iran with a clear choice about the path forward. That's our focus, not thinking of who to blame," said a US official on condition of anonymity.
"We are not naive about what Iran is trying to do by building its nuclear program while talks proceed at snail's pace. We are adapting our posture to Iran's actions even as we seek - in good faith - to try to revive the JCPOA," he said.
"At some point we may conclude with our partners that it no longer is worth saving the JCPOA. We are not there yet. But ... it would be wrong to assume our policy is static."