Now comes the hard part. Having defied many predictions that the P5+1 talks on Iran's nuclear program would fail to reach an agreement, now we move to the approval and implementation phase.

A skeptical US Congress must vote on the Iran deal in September. Republicans are vehemently against the accord, and even some Democrats, fearful of Iran's threat to Israel, are reluctant to support Obama.

But to reject the Iran deal, Congress needs a two-thirds majority to avoid a presidential veto, which Barack Obama has promised if the accord is rejected. While the vote is expected to be close, it is likely that Congress will fall short of the 67 percent majority required for the treaty to be rejected.

Critics point to the 10 and 15 years expiration dates for restrictions on parts of Iran's nuclear fuel cycle activities and the lifting of an embargo on conventional and missile sales after five years as flaws in the accord. Some also fear Iran will cheat and question the effectiveness of IAEA verification, even though it is 24/7 surveillance of all declared facilities.

Skeptics see a nuclear deal as only a delay in Iran's path to eventually building a nuclear weapon, with the lifting of sanctions providing Tehran more resources to expand its influence in the region.

 Perhaps. But diplomacy is the art of the possible and necessarily involves compromise. And it is equally possible that the precedent of reaching a deal with its hated enemy, the US, might change Iran's behavior and open up possibilities of a US-Iran rapprochement.

Congressional critics want to reject the deal, impose tougher sanctions and renegotiate the deal. But this was clearly not possible.

The Iran deal was made possible by US-China cooperation and also the agreement of Russia and the Europeans. What some insular-thinking critics fail to understand is that if the US rejects an agreement negotiated in good faith with its partners, US credibility will be destroyed.

The Europeans, China and Russia would blame Washington and likely normalize their ties with Iran.

The erosion of US credibility would also spill over into US-China relations. How can Beijing and Washington move forward on complex issues from a bilateral investment treaty to cyber security, if the US position is not credible and can be reversed by Congress? The US debate reflects Washington's inability to adapt to a world of diffused wealth and power as much as it does the details of the Iran accord.

Nonetheless, if approved, there are bound to be disputes arising over implementation of the accord. Already, there are US reports that Tehran is "sanitizing" its nuclear plant at Fordow to create difficulties for the IAEA to discover past nuclear activities.

But Iran has powerful incentives to implement the accord. The lifting of sanctions will revive its troubled economy, which President Hassan Rouhani was elected to fix, release some $60 billion in frozen assets, ramp up constrained oil and gas exports and open much needed foreign investment in its energy sector.

In truth, neither critics nor supporters really know what the consequences of the nuclear deal will be. But it may be an important milestone in Sino-US cooperation in the Middle East. Taken together with China's diplomacy to help stabilize Afghanistan and its aid to Iraq to fight IS terrorism, Beijing's P5+1 cooperation in Iran may lay the foundation for deeper Sino-US partnership in the region.

Already, there are rumblings of moves to resolve the brutal, savage civil war in Syria that has killed thousands, created several million refugees destabilizing the region and allowed the IS to grow. Russia has agreed to meet with Syrian opposition forces and Iran has put a peace plan on the table. A resolution of the Syrian mess will not be quick or easy.

But it would be a historic opportunity for the US and China to lead another P5+1 effort.

The author is a senior fellow of the Brent Scowcroft Center on International Security at the Atlantic Council. opinion@globaltimes.com.cn Follow him on Twitter @RManning4.