WASHINGTON (NYTIMES) - When Secretary of State Mike Pompeo asked Mr William Taylor, a highly respected former diplomat, to become America's top diplomat in Ukraine in June, Mr Taylor initially hesitated.

He left government after decades of service in 2009, although he had remained involved in the affairs of Ukraine, a country that had particular meaning for him since he spent three years as the United States ambassador there more than a decade ago in the administration of President George W. Bush.

But he was concerned about the way President Donald Trump's ambassador to Ukraine had been pushed out of her job under suspicious circumstances. So was his wife, who urged him not to take the job, he said in his congressional testimony on Tuesday (Oct 22).

A conversation with a person he described as a trusted Republican mentor who had served in government changed his mind.

"If your country asks you to do something, you do it - if you can be effective," he recalled his mentor saying.

Now Mr Taylor, 72, is at the centre of the scandal engulfing the Trump administration over charges that the President tied American aid to a country fending off Russian aggression to the investigation of his political opponents.

His testimony on Tuesday to committees pursuing an impeachment inquiry against Mr Trump included damning charges of linkage and misleading accounts by Trump administration officials.

A West Point graduate who served as an Army infantry officer leader and company commander in Vietnam and Germany, Mr Taylor is among the country's most experienced diplomats and has served in every administration of both parties since 1985.

Former officials of both parties described Mr Taylor in glowing terms and suggested that his credibility would be difficult for Mr Trump's allies to question.

"Ambassador Bill Taylor is a person of integrity with a strong, ethical base," said Mr R. Nicholas Burns, a former undersecretary of state in the Bush administration. "I would also describe him as a true patriot. His entire professional life has been in service to the US."

"You couldn't ask for a more credible, universally respected, upright public servant to testify on the facts of this case," said Mr Stephen Sestanovich, who served as the State Department's ambassador-at-large for the former Soviet Union under president Bill Clinton and has travelled in Ukraine with Mr Taylor.

"You want to go against Bill Taylor, you've got the whole city against you," Mr Sestanovich added.

Mr Taylor served as ambassador to Ukraine from 2006 to 2009. The country has remained a primary interest.

"Ukraine is special for me," he said on Tuesday, adding that he believed in the "profound importance" of the country to America's security.

He is also intimately familiar with US assistance programmes to former Soviet republics like Ukraine.

From 1992 to 2002, he was coordinator of US assistance to the former Soviet Union and Eastern Europe.

Former officials say that would have given him a strong understanding of the US$391 million (S$533 million) military aid package that the Trump administration delayed this summer as Mr Trump demanded that Ukraine's government pursue an unfounded theory into whether the country interfered in the 2016 election and examine the role of former vice-president Joe Biden's son Hunter Biden in a natural gas company there.

During the Obama administration, Mr Taylor's focus at the State Department was the Middle East, overseeing assistance to Egypt, Tunisia, Libya and Syria at the State Department. He also served in Jerusalem, working on issues related to the Israeli-Palestinian conflict.

Before accepting the job from Mr Pompeo, he was the executive vice-president of the US Institute of Peace in Washington.

"He's just straight-up," said Mr Strobe Talbott, who served as deputy secretary of state under Mr Clinton from 1994 to 2001, focusing on the former Soviet Union, and who encountered Mr Taylor in Ukraine in the 2000s.

"He's courageous. He was just the model of a diplomat, and he had no problem speaking his own mind to his superiors."

Mr Steven Pifer, a former US ambassador to Ukraine from 1998 to 2000 who is now a research fellow at Stanford University, said that he had known Mr Taylor for 25 years and often worked closely with him in Ukraine.

"If Bill Taylor says it happened, it happened," Mr Pifer said.