The agreement to curtail Iran's nuclear programme was hailed by President Barack Obama and other world leaders as a step toward stability in the Middle East.
But there are already competing theories about whether it will help defuse other disputes, or lead hardliners to dig in and use the expected jolt to Iran's economy to escalate long-running proxy wars.
US officials have sought to reassure Middle East allies who fear that the lifting of sanctions on Iran will lead to an economic surge that would enable Tehran to increase its support for militant groups.
On Thursday, Obama expressed hope the deal might result in conversations on other subjects with an Iran that is "less aggressive, less hostile, more co-operative". But he also voiced significant doubt.
"Will we try to encourage them to take a more constructive path? Of course," he said. "But we're not betting on it."
Republicans have criticised the agreement as likely to embolden Iran in its competition with Israel, Saudi Arabia and other US allies. Even within the Administration, there are widely divergent views on how the deal might affect stability in the Middle East.
Obama, for whom the agreement is a signature foreign policy accomplishment, has made the case that reopening Iran's shattered economy will strengthen moderates in the country and push hardliners away from militant activities that prompted broad, international penalties.
Vali Nasr, a former State Department official who serves as dean of Johns Hopkins University's School of Advanced International Studies, urged the Obama Administration to use the nuclear deal to foster more open dealings with Iran over issues such as Isis (Islamic State), the militant group that now controls much of Iraq and Syria.
"We're not in the Arab world of pre-2011 where you have all these stable regimes that are our friends, and even those that are not our friends have control of their territory," Nasr said.
"We're now in an era in the Middle East that is orders of magnitude more complicated. We have to take stock of the reality, rather than focusing only on what Iran is doing."
Others, however, fear the agreement might prompt hardliners in Iran who are worried about a loss of standing to reassert themselves by intensifying support to Shia militias in Iraq or even endorsing attacks on the expanding US presence there.
The US has about 3500 troops in Iraq as the White House broadens its campaign against Isis.
The most religiously conservative elements in Iran have historically held sway over the Quds Force, the foreign military wing of the Iranian Revolutionary Guard Corps, and are most likely to oppose the nuclear deal.
Speaking to lawmakers last week, Obama's top military aide, General Martin Dempsey, cited an array of "malign activities" that Iran might continue. Among them are weapons trafficking, cyberattacks and the use of marine mines.
Iran also continues to hold Americans, including Washington Post correspondent Jason Rezaian, on espionage and other charges.
Obama said on Thursday that "our diplomats and our teams are working diligently to try to get them out", but said the negotiations could not be linked to their release.
The US has engaged in its own cyber-sabotage campaign to derail Iran's nuclear programme. It is unclear whether such efforts will now be suspended because of the agreement, but many expect US espionage efforts against Iran to intensify to monitor compliance with the accord.
Iranian officials have chided Israeli Prime Minister Benjamin Netanyahu for his fervent opposition to the nuclear deal, which many in Iran greeted as a tentative but potentially significant turning point in the country's long-standing and costly conflict with the West.
But the agreement also prompted more cynical praise from Iran's regional allies.
In Syria, Assad struck an emphatically expectant note.
"We are confident that the Islamic Republic of Iran will support, with greater drive, just causes of nations and work for peace and stability in the region and the world," he said.
Thousands of Iran-backed Hizbollah militants are fighting alongside Syrian forces, and Assad recently ratified a US$1 billion ($1.5 billion) line of credit from Tehran.
In Iraq, Tehran's influence appears to be at an all-time high, as Iranian-equipped Shia militia groups have helped reverse gains made by the Sunni-dominated Isis.
Iranian-backed militias killed at least 500 American troops in the Iraq war. But in a measure of how political turbulence has scrambled traditional alignments, the US and Iran are now wary allies in the campaign against Isis.
Forces from Iranian-backed militias have massed around the Iraqi city of Fallujah, US officials say, complementing the offensive started this week by American-supported Iraqi troops around nearby Ramadi.
Defending the deal, American officials have pointed to plans to extend a United Nations embargo on arms sales to Iran. The US also has worked to block weapon shipments to proxy groups and will retain sanctions on individuals supported by Iran.
At the same time, US officials are working to bolster joint security measures for Persian Gulf nations, whose leaders Obama gathered at Camp David in May. But enacting such measures has been slow.
Obama played down the potential for Iran to increase its proxy campaigns after the nuclear accord. With sanctions removed, he said, "I think that is a likelihood that they've got some additional resources. Do I think it's a game-changer for them? No."
Israel blasts British view
Israel's Prime Minister Benjamin Netanyahu yesterday delivered a sharp rebuff to British Foreign Secretary Philip Hammond over the Iran nuclear deal, publicly dismissing Hammond's assertion that Israel would have been unhappy with any agreement.
In a tense exchange that reflected the gulf between Israel and the six world powers who negotiated with Tehran, Netanyahu rejected Hammond's efforts to sell the pact, while tackling criticisms by Hammond.
"Israelis know better than anyone else the cost of permanent conflict with Iran and it is wrong to suggest that Israel wants such an outcome. We seek a genuine and effective diplomatic solution," Netanyahu told Hammond in Jerusalem.
"The alternative to this deal is not war. The alternative is a better deal that would roll back Iran's military nuclear programme and tie the easing of restrictions on Iran's nuclear programme to changes in Iran's behaviour."
On Thursday, Hammond told the House of Commons that Israel opposed any accord with Tehran and would prefer permanent conflict.
- Daily Telegraph UK
