Diplomats from Iran, the United States and five other powers gathered in Vienna last week to try to revive President Barack Obama's 2015 deal limiting Tehran's nuclear activities.

It did not go well.

Iran's new hard-line government showed up with maximalist demands, insisting the United States lift all its economic sanctions before Tehran takes any steps toward curbing its uranium enrichment.

And the Iranians went further: They said they wanted to reopen draft agreements that their predecessors negotiated only six months ago.

Meanwhile, the U.N.'s nuclear watchdog agency announced that Iran has escalated uranium enrichment at an underground plant in violation of the 2015 deal.

Tehran's actions drew harsh responses not just from the United States, but also from its European allies.

“Iran has fast-forwarded its nuclear program … (and) backtracked on diplomatic progress,” diplomats from Britain, France and Germany said in a statement.

In undiplomatic terms, Iran shot itself in the foot: It shifted blame for any impasse from the United States to itself.

In doing so, it raised a larger, more ominous question: Does Ayatollah Ali Khamenei, Iran's supreme leader, want an agreement at all?

“The Iranians know that compromise will be necessary to make a deal,” said Suzanne DiMaggio, a nuclear arms expert at the Carnegie Endowment for International Peace. “But it's unclear whether this new group of hard-liners is willing and able to get there.”

A little history is in order.

Obama and the leaders of China, Russia, Britain, France and Germany negotiated the 2015 deal to prevent Iran from acquiring the ability to assemble a nuclear weapon. Iran says its nuclear program is peaceful, but it has enriched some uranium to a level that is mainly useful as a step toward a bomb.

Under the deal, the United States and other countries promised to lift economic sanctions in exchange for strict limits on Iran's nuclear activities.

Iran initially complied, closing nuclear facilities and limiting uranium enrichment. But the economic benefits fell short of expectations: Western banks and businesses didn't flood Tehran with investments.

Then, in 2018, President Donald Trump denounced the agreement as “the worst deal ever,” walked away from it, and imposed crippling economic sanctions.

For more than a year, Iran continued to obey the pact's nuclear limits, hoping other countries would undo Trump's sanctions. But the sanctions never came off, and in 2019 Iran began enriching uranium beyond the limits.

Under the deal, Iran's “breakout time” — the period it would need to build a nuclear weapon — had lengthened to about a year, a delay U.S. military officials said would give them time to react.

After Trump withdrew from the agreement, Iran reduced its breakout time to roughly a month.

Where to go from here?

One sensible approach for the U.S. and its allies would be a step-by-step process, with sanctions coming off gradually as Iran moves back toward compliance with its 2015 commitments. But Iran has rejected that.

If Iran continues to escalate uranium enrichment, the Biden administration and its allies may opt for what some diplomats call “Plan B” — new economic sanctions and tougher enforcement of existing ones.

Iran says it can't be sure the next U.S. president will honor any commitments. So it will be difficult to conclude a full agreement before the 2024 election.

But a delay in timing may suit Biden too. Republicans are likely to call any deal a sellout; the president probably doesn't want a debate over concessions to Iran right now.

Without an agreement that constrains Iran from moving closer to possessing a nuclear weapon, Biden or his successor may face an unpalatable choice: Accept an Iran with the ability to assemble a nuclear weapon or go to war.

It's surely worth taking more time, offering more flexibility and enduring more rounds of unproductive talks to prevent that moment from arriving.

Doyle McManus is a columnist for the Los Angeles Times.