The deal will also be a rare legacy for Barack Obama's administration. The mid-term election in 2014 resulted in Republican control of both the House and the Senate. As a result, Barack Obama will find it hard to push forward his domestic agenda. A deal with Iran and the détente with Iran in the aftermath of the deal will be another legacy for the administration in foreign policy in addition to normalizing relations with Cuba.

Though questioned and challenged at home, the deal will prove to be an achievement that stands the test of time. Neither Iran, one of the most important regional powers, nor the U.S., the most important global power, can afford to be hostile to the other side after thirty six years. Hostile policy has consumed huge strategic resources on both sides and wasted too many opportunities for cooperation.

The deal is also a big personal achievement for John Kerry, U.S. secretary of state. Kerry regards himself as an expert on Middle East issues. Since being appointed as secretary of state, he has been concentrating his full energy on various Middle East issues. He even used to work on the Palestine-Israel peace process, Syrian issue and the Iran nuclear issue.

As he focused too much on these issues, he has been dubbed as assistant secretary of state on Middle East issues rather than secretary of state taking care of overall American foreign policy.

Despite frustrations in Palestine-Israel negotiation and Syrian issue, the nuclear deal will prove to be worthy of his efforts on Middle East issues.

Besides being a success, the deal is also a big challenge for Barack Obama's administration. The first and the biggest challenge will come from Congress. Though Barack Obama has proved to be able to defeat the rivalry at the negotiating table, he cannot necessarily win over his opponents at home.

He could demonstrate his presidential power by vetoing the decision of Congress after reviewing of the deal for 60 days, but he might not be able to persuade Congress to remove the sanctions on Iran's oil and financial sectors. That is part of the commitments that the United States made in the deal. He could make concessions in other areas in exchange for that, but the concessions would have to be huge. Then what bargaining chip could he use to make that trade?

The second major challenge comes from abroad. As mentioned, the deal has caused tremendous dissatisfaction from Saudi Arabia and Israel. Both countries are still crucial allies of the United States in the region. Barack Obama will have to do something to pacify their anger. Sales of more advanced weapons might be one choice. But it is not known whether Saudi Arabia or Israel would buy them.

Last but not least, the Iran deal also signifies the decline of the United States in the Middle East. The single most influential external power of the U.S. used to be its hard power to push forward its agenda. But this time it had to depend on diplomacy instead of military means to resolve a thorny issue. It even had to give up some of the objectives it had stood for at the very beginning. It might mean something good, but also means that the U.S. is really declining.

The author is a columnist with China.org.cn. For more information please visit: http://china.org.cn/opinion/jinliangxiang.htm

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