William Taylor, the US Diplomat who said it was crazy to withhold security assistance in Ukraine messages will be the latest to join the list of people to testify in front of the Congress for impeachment inquiry against President Donald Trump.

Taylor, the retired career civil servant has emerged as an unlikely central player in the events that are at the heart of the impeachment inquiry.

Taylor was tapped to run the US Embassy in Ukraine after the administration abruptly ousted the ambassador. He was then drawn into a Trump administration effort to leverage US military aid for Ukraine. And then he grew alarmed.

"I think it's crazy to withhold security assistance for help with a political campaign," he wrote at one point in excerpts of text messages released by impeachment investigators in Congress.

And now, members of Congress will hear directly from Taylor.

Democrats have levelled charges of misuse of power against Trump. They have said that that according to a transcript of a call that Trump made with Ukraine, he asked Ukrainian President Volodymyr Zelensky to investigate Joe Biden and his son Hunter Biden for corruption. Joe Biden could be Trump's opponent in the 2020 presidential race. Trump allegedly threatened to withhold military assistance worth $400 million, if 'deliverables' were not found in the investigation.

Taylor, in a follow-up to the call, exchanged texts with two of Trump's point men on Ukraine as they were trying to get Zelenskiy to commit to the investigations before setting a date for a coveted White House visit.

In a text message to Gordon Sondland on Sept. 1, Taylor bluntly questioned Trump's motives: "Are we now saying that security assistance and WH meeting are conditioned on investigations?" Sondland, US ambassador to the European Union, told him to call him.

On October 18, Acting White House Chief of Staff Mick Mulvaney suggested that the Trump administration held up the military aid to Ukraine in part because the president wanted it to investigate an unproven conspiracy theory about Ukraine's involvement in the hack of the Democratic National Committee (DNC) server in 2016. While the President has justified his asking Ukraine to investigate Biden, Trump admin has maintained that releasing military aid was not conditional to Ukraine's investigation on the Bidens.

The former Army officer is scheduled to testify behind closed doors Tuesday in an inquiry trying to determine if Trump committed impeachable offences by pressing the president of Ukraine into pursuing information that could help his campaign as Trump withheld military aid to the Eastern European country.

Taylor had been serving as executive vice president at the US Institute of Peace, a nonpartisan think tank founded by Congress when he was appointed to run the embassy in Kyiv after Ambassador Marie Yovanovitch was removed before the end of her term following a campaign against her led by Trump's lawyer, Rudy Giuliani. Yovanovitch, in a statement, had said that she had been ousted on "unfounded and false claims by people with clearly questionable motives". She had further said that it was "Equally fictitious is the notion that I am disloyal to President Trump."

Taylor was chosen for the post because he was among only a handful of former officials with experience in Ukraine who would be perceived as neutral by local officials and wouldn't raise objections at the White House, according to a colleague.

"It was a very short list, but Bill was at the top of it," said the colleague, who was not authorized to speak publicly on condition of anonymity. "We were very grateful he agreed to do it." Taylor, who had served as US ambassador to Ukraine from 2006 to 2009, was welcomed back to Kyiv as a steady hand. Taylor was involved in diplomatic efforts surrounding several major international conflicts. He served in Jerusalem as US envoy to the Quartet of Mideast peacemakers.

He oversaw reconstruction in Iraq from 2004 to 2005, and from Kabul coordinated the US and international assistance to Afghanistan from 2002 to 2003.

He arrived in Kyiv a month after the inauguration of Ukraine's new president, prepared to steer the embassy through the transition. He was most likely not prepared for what happened next.

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