The real test, caution some, is whether Iran follows through on the tentative nuclear deal that would effectively prevent Tehran from developing a bomb.
A nuclear reactor was sold to Iran as part of President Eisenhower's "Atoms for Peace" program. But even then the US had concerns about what might happen if the Shah fell to 'domestic dissidents.'
The talks in Geneva were historic, but some experts say Iran gained prestige without having to concede much.
Iran may have built a shadow infrastructure to provide the raw material for building nuclear weapons, experts say.
At a midday break, diplomats said the tone has been "civil" but a US official says Iran's delegation lacks the 'cohesion and confidence' to make a deal.
The US is prepared to expand and toughen existing sanctions if Iran does not reverse course on its nuclear program.
Beijing is against sanctions as a matter of principle – and because of recent multi-billion dollar energy deals with Iran.
After last week's call for 'crippling sanctions' against Iran, Israel has adopted an 'eloquent silence' on the issue while it waits to see how Thursday's historic nuclear talks go.
Iran said Tuesday that it would open the site near Qom to international inspectors. But it didn't promise much cooperation.
The show of force ahead of nuclear talks is a well-tested strategy. But only 1 in 10 Iranians support nuclear weapons, and many question the government's legitimacy after June elections.