BEFORE NEGOTIATIONS on Iran’s nuclear program resumed last week, Iranian officials projected an unlikely tone of optimism about the chances that a comprehensive accord could be struck by a July 20 deadline. U.S. officials, who were much were cautious, were quickly proved right: When the round of talks ended Friday in Vienna a senior American official spoke of “great difficulty” while Iran’s representative said “we failed.”

The differing initial rhetoric reflected a more substantive imbalance that may be to the advantage of the United States and its allies. Iranian President Hassan Rouhani feels more urgency to strike a deal than does President Obama. The Iranian economy is still suffering from international sanctions; predictions by Israeli leaders that controls on trade and investment would crumble after an interim agreement offered Tehran partial relief have not been borne out. Elected on the promise that he could end Iran’s isolation and revive the economy, Mr. Rouhani is under pressure to deliver. With Iran’s nuclear work mostly frozen, Mr. Obama can afford to wait. A six-month extension of the talks provided for in the interim deal might even be in his interest.

Whether that discrepancy can be used to leverage the major concessions Iran must make for a workable agreement is anyone’s guess — though this week’s talks were a bad omen. A senior U.S. official said “we do not know if Iran will be able to make the tough decisions they must.” Chief among those is steps that would make it impossible to produce the material for a bomb in less than “six months to a year,” a time frame mentioned by Secretary of State John F. Kerry in recent testimony to Congress.

That period may sound too short to Israel and to many members of Congress. But it would require Iran to dismantle the majority of the 19,000 centrifuges it has installed in two different enrichment plants, one of them buried deep underground. At least in public, Iranian officials have so far been saying that they not only won’t destroy existing nuclear infrastructure, but that they also intend to add thousands more centrifuges and to introduce a new generation of faster machines.

Another key issue is ballistic missiles. U.S. officials said the agreement must address U.N. Security Council resolutions on Iran, which cover any missile capable of carrying a nuclear warhead. But Iranian Supreme Leader Ali Khamenei last week called demands for missile limits “stupid and idiotic.”

If these yawning gaps can somehow be bridged, a final hurdle will be determining how long any limits on Iran’s nuclear work will last. In the interim agreement, Tehran obtained language saying the controls would eventually expire, and its negotiators reportedly suggested a time frame of just 3 to 5 years. U.S. negotiators will want a decade or more.

In the end there will be a strong check on any concessions made by the Obama administration: If Congress or Israel are dissatisfied, they may be able to scuttle the deal. Iranian hardliners will have a say, as well. Considering the challenge of constructing a compromise that satisfies the Revolutionary Guard and Republican senators, it’s no wonder that optimism seems out of place.