WASHINGTON - As chief executive of Exxon Mobil, Rex Tillerson tracked diplomatic relations between Iran, the United States and other Western powers, watching for developments that might open some of the last easy-to-access oil and gas deposits in the world after decades of revolution, war and international sanctions.

Now, as secretary of state, Tillerson will soon help decide whether to stick to an agreement that has begun that opening and held out the promise of a bonanza for U.S. and European energy companies, many with significant operations in Houston and Texas.

Tillerson's past and present worlds are set to collide as critical deadlines loom for the United Nation's deal that lifted some economic sanctions in exchange for the Islamic Republic curtailing its development of nuclear technology. When former President Barack Obama signed onto the deal early last year, he did so conditionally, with regular reviews scheduled every 4 to 6 months to determine whether the U.S. would stick to the agreement.

The next round of reviews begins this month, raising the question of whether the Trump administration will uphold an accord that President Donald Trump recently called "terrible" on his Twitter feed and has signaled he plans to renegotiate.

Those statements have fueled anxiety not only in Iran - where President Hassan Rouhani has promised the nuclear deal will revive an economy crippled by years of sanctions - but also among oil and gas companies examining multibillion dollar development deals there. That includes Shell, Total and Schlumberger, which together employ thousands of workers in Houston, according to a list released by Iran's national oil company earlier this year.

"There's a lot of interest, but there's also a lot of trepidation," said Suzanne Maloney, a former State Department adviser and now a senior fellow at the Brookings Institution. "So far, Tillerson has been restrained on this issue."

A spokeswoman for the State Department said the Trump administration is conducting a "rigorous" review of the Iran nuclear deal. "While that review is underway, the United States continues to uphold its commitments," she said.

Complex situation

The nuclear deal was the culmination of a long international campaign to get Iran to back off nuclear development, which Iranian leaders said was simply for electricity generation and other commercial applications but U.S. intelligence officials feared was aimed at developing weapons. Obama's decision to sign onto the agreement drew condemnation from Republicans, supporters of Israel and other critics.

Finding a course forward in Iran is among the most complicated items on the State Department's to-do list, said Joe Barnes, a former State Department official who served in the Middle East and is now a fellow at Rice University's Baker Institute for Public Policy. Tighten sanctions again and you risk a relatively moderate Iranian government walking away and hard liners in Tehran gaining momentum; relax them too much and you lose leverage on a host of separate and equally import diplomatic issues, including Iran's funding of the terrorist group Hezbollah, human rights abuses and its military involvement in the Syrian civil war on behalf of the dictator Bashar al-Assad.

To this balancing act, Tillerson and Trump must consider the cascading effect that any changes in the terms of the deal would have on relationships with countries from Israel to Russia to Saudi Arabia to China, Barnes said.

"It's not just a deal between the United States and Iran, it's between a number of countries and Iran," he said. "Renegotiating the deal would be immensely complicated."

The potential for a major policy shift is already causing significant anguish within the world's oil and gas industry, of which Tillerson, as Exxon CEO, was long part of the small core of de-facto leaders. Already U.S. companies like Exxon, Chevron and ConocoPhillips, which remain constrained under sanctions outside the nuclear deal, are falling behind their European counterparts, which are in talks for development deals with the Iranian national oil company.

Just 12 months ago, while still at Exxon Mobil, Tillerson said of Iran during a television interview, "We'll wait and see if things open up for U.S. companies. We would certainly take a look because it's a huge resource-owning country."

A spokeswoman for Chevron said she could not discuss Iran directly, but "economic returns, stability of the investment climate and sanctity of contract are central to any decisions we make." Exxon declined to comment. ConocoPhillips did not respond to requests for comment.

Significant opportunity

Iran's oil and gas fields rank as the fourth and second largest proven reserves in the world, respectively, according to the Department of Energy. And after years of being in "the penalty box," those fields are ripe for new investment and updated equipment, said Fadel Gheit, senior energy analyst at the investment firm Oppenheimer & Co. That could one day mean significant opportunity for Houston's oil and gas equipment supply chain, along with service companies like Halliburton, whose expertise would be in demand.

Gheit pointed to Iraq, where many U.S. companies, including Halliburton, gained significant business as the nation rebuilt its oil industry and increased production after the 2003 war that toppled the dictator Saddam Hussein.

"We've seen what's happened in Iraq, and if sanctions are lifted and relations are normalized Iran has much greater upside potential," he said. "But you don't want to sink capital there and get into an impasse situation. Because you have to recognize it's going to be a big investment, a multiyear, multibillion-dollar investment."

In November, the French oil major Total said it signed a $2 billion deal to develop what is considered the world's largest natural gas field along the Iranian coast of the Persian Gulf. Then three months later, the Trump administration announced a new round of sanctions targeting 25 Iranian individuals and companies, following a ballistic missile test by the Iranian military. Within days Total CEO Patrick Pouyanné told journalists in Paris that if the United States leaves the U.N. nuclear deal, "We'll not be able to work in Iran."

As the deadline for Tillerson and Trump to make a decision on the nuclear deal nears, pressure is ramping up. Even before Trump took office, the Iranian government protested that the United States, despite lifting some sanctions, is still maintaining terrorism-related sanctions that largely prohibit American financial institutions from lending or moving money on transactions that involve Iranian companies.

In Washington, Democrats and Republicans alike maintain their long held position that as long as Iran continues to fund groups like Hezbollah, there is no room for any appeasement. Last month a bipartisan group of senators, including the leaders of the Senate Foreign Relations Committee, Sen. Bob Corker, R-Tenn., and Sen. Ben Cardin, D-Md., as well as Sen. Ted Cruz, R-Texas, introduced legislation that would further expand sanctions against Iran- though not so far as to violate the terms of the nuclear deal.

Under that agreement, the United States was only required to lift those sanctions imposed for nuclear development, not those related to issues like terrorism and human rights abuses.

"The conventional wisdom in Washington is (Iran is) on the march;" Maloney, the former State Department adviser, said. "The feeling is we need to be much more aggressive in pushing them back from gaining the upper hand in the Middle East."

Low profile

Since taking over the State Department last month, Tillerson has maintained a conspicuously low public profile compared to predecessors like John Kerry, who served under Obama, and Condoleezza Rice, who served under President George W. Bush. But on one of the rare occasions in which he has made a public statement, he quickly found what a sensitive issue Iran has become.

Offering a seemingly benign greeting on Iran's new year's holiday Nowruz, Tillerson said, "We look forward to celebrating this new spring together." The National Iranian American Council immediately questioned the statement, asking how Iranian Americans were supposed to celebrate together considering the travel ban Trump ordered on Iran and other predominantly Muslim countries earlier this year. Federal courts have blocked the ban, however.

At a news conference State Department spokesman Mark Toner said it was inappropriate to connect the statement to the travel ban.

"This was a recognition of an important event for Iranians and Iranian Americans," he said, "and nothing more."