As chief of ExxonMobil, Rex Tillerson honed many of the skills inseparable from the exercise of high-level diplomacy: navigating complex geopolitics and pushing tough negotiations with friends and foes around the world.

The question now facing the 64-year-old Texan, as President-elect Donald Trump's nominee for secretary of state, is whether he can bring those skills to serve American interests, and break free from the oil industry that shaped him over a lifetime.

As the Republican Trump himself put it, in announcing his choice of the chief executive for top diplomat on Tuesday, "his relationships with leaders all over the world are second to none."

One of those relationships -- with Vladimir Putin, who awarded him Russia's "Order of Friendship" in 2013 -- was likely a key asset in Trump's view, as he pushes for a detente with Moscow.

But Tillerson's Russia ties have raised hackles across the US political spectrum, and threaten to severely complicate his approval by the Senate, against the backdrop of intelligence indicating Moscow interfered to try to sway the election for Trump.

Senator John McCain was among those voicing concern: "Vladimir Putin is a thug, bully and a murderer, and anybody else who describes him as anything else is lying," he said ahead of Tillerson's nomination.

And if Trump sees Tillerson as a dynamic deal-maker whose contacts are a precious asset for his administration, environmentalists have voiced serious concern at the prospect of an oil industry veteran representing the United States in global climate negotiations.

- 'Parallel quasi-state' -

Born in Wichita Falls, Texas, Tillerson -- a lifelong Boy Scout -- joined ExxonMobil in 1975, as a young engineer fresh out of college, and has never worked anywhere else, steadily rising through the ranks to become CEO in 2006.

After a decade spent overseeing the company's business activities in more than 50 countries, the silver-haired married father of four had been due to retire in March.

Steve Coll, author of a 2012 investigative book about ExxonMobil, wrote in The New Yorker that Tillerson's life was profoundly influenced by the oil giant, an organization that promotes almost all its leaders from within.

Coll describes how ExxonMobil perceives itself -- as a transnational power independent of the US government, and possessed of its own foreign policy that seeks to create the best conditions for oil and gas production.

In furthering that policy, Tillerson has effectively been "running a parallel quasi-state," Coll says, "fashioning relationships with foreign leaders that may or may not conform to the interests of the United States government."

That included defying the State Department to cut an oil deal in Iraqi Kurdistan, or working in partnership with authoritarian regimes despite a stated company policy of promoting human rights.

But Coll's investigation also describes Tillerson's record at ExxonMobil as one of professional integrity.

Tillerson's relationship with Putin is believed to have been central to his success as CEO.

- Easing of sanctions? -

The two met in the 1990s when Tillerson was supervising a project on Sakhalin Island, and strengthened their ties when Putin took power following Boris Yeltsin's resignation in December 1999.

Over the years, Tillerson "has had more interactive time with Vladimir Putin than probably any other American with the exception of Henry Kissinger," said John Hamre of the Center for Strategic and International Studies (CSIS).

The friendship was crowned by a historic agreement Exxon signed in 2011 with Russian public energy giant Rosneft to explore and drill in the Arctic and Siberia -- a project facilitated by the melting of Arctic ice due to global warming.

The deal, at first valued at $3.2 billion, could potentially generate a hefty $500 billion depending on oil discoveries -- but has been put on hold by Western sanctions against Russia.

Tillerson has been a vocal opponent of sanctions as a foreign policy instrument and would, as secretary of state, be in a position to benefit his lifelong employer by advocating an easing of sanctions against Russia.

"We always encourage the people who are making those decisions to consider the very broad collateral damage of who are they really harming with sanctions," he argued at a 2014 shareholders' meeting.

Tillerson's position as the holder of some $150 million in Exxon shares -- liable to fluctuate based on his actions as top diplomat -- creates a significant potential for conflict of interest, although he would presumably be required to divest his holdings.

- 'No friend of the planet' -

Among the burning issues awaiting Tillerson beyond Russia are ties with China, the protracted Syrian conflict, Trump's vow to review the Iran nuclear deal and to pull out of both the TPP Pacific rim trade deal and the Paris climate agreement.

Beyond his advocacy for free trade, Tillerson's broader political views are largely unknown, but environmentalists are raising a red flag over ExxonMobil's past activism on the issue of climate change.

Several US states, supported by environmental activists, are suing the oil giant for allegedly deceiving the public about the role of fossil fuels in global warming.

Tillerson himself is credited with steering ExxonMobil towards public acceptance of the science of climate change. He came out in favor of a carbon tax in 2009, which his predecessor Lee Raymond fought, but he also resisted cutting investment in the search for new oil wells.

Environmental group 350.org dubbed the nomination "unfathomable."

"We cannot let Mr. Trump name the world's largest oil company in charge of our international climate policy. Mr. Tillerson may be a friend of Mr. Putin, but he is not a friend of the planet," it charged.

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