On paper, Kurt Volker's job in the Trump administration was to support Ukraine and help end a war started by Russia in the east of the former Soviet Republic. Volker is now caught up in a political battle at home over President Trump's efforts to get Ukraine to dig up dirt on former Vice President Joe Biden and his son Hunter.

Volker will be deposed Thursday behind closed doors as part of the House of Representatives' impeachment inquiry into President Trump.

Volker, 54, was a career diplomat who focused on Europe and was tapped by then-President George W. Bush in 2008 to serve as the U.S. ambassador to NATO, a position he held for less than a year.

By the time Russia annexed Crimea in 2014 and sent troops to foment an uprising in eastern Ukraine, Volker was out of government, running the McCain Institute, a think tank in Washington run by Arizona State University. He was critical of the Obama administration's approach to Russian President Vladimir Putin's aggression.

"The most frequent phrase you hear out of mouths now is there is no military solution, and I think we just have to reject that," he told NPR in a 2015 interview. "We are seeing a military solution play out before our eyes on the ground in Ukraine, and it happens to be one that we don't like. It's Putin's military solution."

Volker returned to the State Department in July 2017 when he was tapped by then-Secretary of State Rex Tillerson to serve as U.S. special representative for Ukraine negotiations.

Andrew Weiss, of the Carnegie Endowment for International Peace, said Volker was an unlikely fit in the Trump administration.

"It was indicative of just how hard it was to get credentialed middle-of-the-road or right-of-center Republicans to serve in this administration," Weiss said. "So there was a real shortage of talented experienced people coming in. Kurt was one of the exceptions to that."

Kurt was appointed with a specific role in mind, Weiss said: halting the conflict in eastern Ukraine. But that mandate broadened over time.

"He ended up having a far wider portfolio that involved running U.S. policy on Ukraine writ large," Weiss said.

For one thing, Volker had to unify the Trump administration's position on Ukraine.

"Donald Trump came into office with a very bad attitude about Ukraine as a candidate," Weiss added. "[Trump] repeatedly talked about how Crimea would have been happier being part of Russia and how basically Ukraine was secondary to his all-important goal of resetting relations with the Kremlin."

Volker, though, made sure that the Trump administration sided with Ukraine when it comes to Crimea. He also oversaw a change in policy, with the U.S. now providing anti-tank systems and other defensive weapons to Ukraine in its conflict with Russian-backed forces in Ukraine's east.

Volker's work with the State Department was a part-time, volunteer job. He held on to his position at the McCain Institute and BGR, a powerful lobbying firm that represents Ukraine and Raytheon.

Raytheon manufactures the Javelin missiles that were part of the Trump administration's military aid package to Ukraine. The work raised questions about potential conflicts of interest. Volker did have an ethics agreement approved by State Department lawyers and recused himself from some of BGR's work. He has not been accused of violating any conflict-of-interest rules.

Instead, it is Volker's work with Trump's personal lawyer, Rudy Giuliani, that is now under investigation. Sen. Chris Murphy, D-Conn., who sits on the Senate Foreign Relations committee, tweeted on Sept. 27 that Volker has "a well deserved reputation for fairness, toughness and integrity."

But, Murphy told NPR, "my esteem for Kurt frankly makes me even more disappointed that he has become part of this mess and perhaps facilitated the corruption of the State Department."

Volker resigned as special representative on Sept. 27. Volker offered no public explanation, and Murphy — who recently visited Ukraine — is calling on him to speak up now.

"I'm glad that Kurt stepped down, and now he needs to fess up to what he knows and what he did," said Murphy.

According to a whistleblower complaint filed last month about an interaction President Trump had with the leader of Ukraine, Volker visited Ukraine's capital of Kyiv a day after the July call between Trump and Ukrainian President Volodymyr Zelenskiy.

Volker "provided advice to the Ukrainian leadership about how to 'navigate' " Trump's demands, according to the complaint. The as-yet unidentified whistleblower, citing multiple officials,claimed that Trump urged Zelenskiy to investigate Joe Biden and his son Hunter.

The complaint is now central to the impeachment inquiry announced by House Speaker Nancy Pelosi, D-Calif., on Sept. 24.

The Carnegie Endowment's Weiss said Volker may have believed he had to "corral the crazy stuff that Giuliani was doing to make sure that it wouldn't contaminate or impede the important work of U.S. foreign policy."

"Donald Trump clearly thought that embracing Giuliani's quixotic quest to get the goods on the supposed Ukrainian interference in 2016 was the priority for U.S. foreign policy," said Weiss. "He had this exactly backwards compared to work Volker and other career officials were doing."

Secretary of State Mike Pompeo is making a different case. He told reporters during a news conference in Rome that Volker was focused on "taking down the threat that Russia poses there in Ukraine," adding that continues despite "all this noise going on."

Volker has not commented publicly on the matter because of the House investigations.

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