In the weeks since Congress launched an impeachment inquiry into President Donald Trump, current and former officials have come forward to testify against the president.

Their revelations show how Trump employed his personal lawyer Rudy Giuliani and other cabinet officials to bend federal agencies to suit his needs.

Chief among those is the US State Department which, according to testimony from current and former officials, was used to circumvent career diplomats and carry out a shadow foreign-policy agenda with respect to Ukraine.

At least eight officials have testified or are scheduled to testify about the extent to which Trump and Giuliani gutted and politicized the State Department.

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In the weeks since House Democrats launched a formal impeachment inquiry into whether President Donald Trump used his public office for private gain, current and former officials have come out of the woodwork to testify against the president.

At times defying orders from the White House itself, these officials' revelations paint a stark portrait of a concerted effort from the highest levels of the Trump administration to leverage US foreign policy for the president's political benefit.

They also show how he employed his personal lawyer Rudy Giuliani and top cabinet officials to bend federal agencies to suit his needs. Chief among those is the US State Department which, according to testimony from current and former officials, was used as part of an effort to circumvent career diplomats and policy experts and carry out a shadow foreign-policy agenda with respect to Ukraine.

Read more: The floodgates are opening as Trump officials publicly defy his orders and more whistleblowers come out of the shadows

At least eight current and former officials have so far testified, or are planning to testify, about how the State Department was used as a vehicle for Trump and Giuliani's political goals:

Marie Yovanovitch, the US's former ambassador to Ukraine.

Fiona Hill, the former White House senior director for Russian and Eurasian affairs.

Michael McKinley, a former senior adviser to Secretary of State Mike Pompeo.

Kurt Volker, the US's former special representative to Ukraine.

Gordon Sondland, the US's ambassador to the European Union.

Bill Taylor, the chargé d'affaires at the US's embassy in Ukraine.

George Kent, the deputy assistant secretary in the State Department's Bureau of European and Eurasian Affairs.

Steve Linick, the State Department's inspector general.

Read more: These are the key players you need to know to make sense of the Trump impeachment inquiry

Former US Ambassador to Ukraine Marie Yovanovitch. Scott J. Applewhite/AP

A politically motivated ouster led to the resignation of Mike Pompeo's top deputy

Perhaps the most significant example of this was Yovanovitch's dismissal.

She was abruptly recalled from her position in May after a tumultuous stint that involved frequent clashes with Giuliani.

Their main disagreement stemmed from her refusal to facilitate the former New York mayor's efforts to use official channels to push the Ukrainian government to dig up damaging information on former Vice President Joe Biden and his son.

"She refused to allow her embassy to be dragged into some sort of effort to concoct dirt for political purposes," a former official told The Guardian.

What Yovanovitch said

Yovanovitch testified to Congress last week that she was removed based on "unfounded and false claims by people with clearly questionable motives" and that the campaign against her was spearheaded by Trump and Giuliani beginning last spring.

She also testified that a senior State Department official told her she was being recalled despite the fact that she had "done nothing wrong."

Yovanovitch's account has been corroborated by several other officials who testified to Congress.

What Fiona Hill said

Hill, who served as the White House's senior director for Russian and Eurasian affairs, praised Yovanovitch when she appeared before lawmakers on Friday.

The New York Times reported, moreover, that Hill saw Yovanovitch's removal as an abuse of the system.

Hill also described how Giuliani's efforts in Ukraine were perceived within the White House, at one point telling lawmakers that former national security adviser John Bolton was so alarmed that he instructed Hill to tell a top White House lawyer he was not involved in the "drug deal" being "cooked up" in Ukraine.

On Wednesday, a former top deputy to Secretary of State Mike Pompeo made the stunning revelation to congressional investigators that Yovanovitch's ouster was the catalyst for his resignation last week.

What Michael McKinley said

McKinley, who served as one of Pompeo's senior advisers, testified that career diplomats were routinely mistreated and sidelined within the department for political reasons.

He said Yovanovitch's dismissal was the last straw for him.

According to a copy of his opening remarks that was obtained by The Times , McKinley testified that after learning last month that Trump slammed Yovanovitch as being "bad news" during a July 25 phone call with Ukraine's president, he tried to get senior State Department officials to publicly defend Yovanovitch for her "professionalism and courage."

McKinley said his request garnered no significant response from the agency, no statement was issued, and he ultimately decided to resign.

Read more: FBI officials were 'rattled' and 'blindsided' by Trump's call for Ukraine to manufacture dirt on Joe Biden

Kurt Volker. AP

Infighting between 3 top diplomats over Giuliani's pressure campaign

Then there's the trio of diplomats — former Special Representative to Ukraine Volker, US Ambassador to the EU Sondland, and the chief US diplomat in Ukraine Taylor — whose explosive text messages to one another revealed just how intricately involved US government officials were in Giuliani's pressure campaign on Ukraine.

What Volker's and Sondland's texts revealed

Volker was instrumental in establishing a channel of communication between Giuliani and Andriy Yermak, a key aide to Ukrainian President Volodymyr Zelensky.

Volker and Sondland exchanged several texts in which they both expressed the understanding that a good relationship between Trump and Zelensky was predicated on the Ukrainian government pursuing "investigations" and "getting to the bottom" of what happened in 2016.

This was a reference to two of Trump's main requests: that Zelensky launch an investigation into Burisma Holdings, the Ukrainian natural gas company whose board Hunter Biden sat on, and that Zelensky help discredit the FBI's finding that Russia interfered in the 2016 election.

Read more: Trump's presidency is disintegrating as he faces his worst 30 days since taking office

What Taylor's texts revealed

Taylor, the skeptic of the group and a career foreign-service officer, sounded the alarm from the start about how the Trump administration was handling the US's relationship with Ukraine's newly elected government.

"Are we now saying that security assistance and WH meeting are conditioned on investigations?" Taylor texted Sondland on August 30. "Call me," Sondland replied.

The messages came shortly after it was reported that the US government was holding up a nearly $400 million military-aid package to Ukraine.

Trump ordered his administration to put a hold on the aid in mid-July, just a few days before a now infamous phone call with Zelensky, during which Trump reminded his Ukrainian counterpart that "we do a lot for Ukraine" and followed up by asking Zelensky to "do us a favor, though," and investigate the Bidens.

Read more: Rudy Giuliani urged Trump to turn over an exiled Muslim cleric to Erdogan, raising concerns that he was lobbying for Turkey

Taylor, who will testify to Congress on Tuesday, continued his criticisms of the president as details spilled out about Trump's conversation with Zelensky.

"As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign," Taylor texted Sondland on September 9.

Five hours later, Sondland came back with a lengthy response.

"I Believe you are incorrect about President Trump's intentions," he wrote. "The President has been clear no quid pro quo's of any kind. The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelensky promised during his campaign I suggest we stop the back and forth by text If you still have concerns I recommend you give Lisa Kenna or a call to discuss them directly. Thanks."

Sondland tries to save face

The EU ambassador distanced himself from the Ukraine controversy when he testified on Thursday, and he told lawmakers the contents of his text to Taylor were relayed directly to him by Trump in a phone call.

"It's only true that the president said it, not that it was the truth," Sondland testified, according to his opening statement. He also contradicted the president's claims denying a quid pro quo and acknowledged that Trump's efforts did, in fact, represent a quid pro quo "but not a corrupt one."

Sondland's attempts to shield himself from scrutiny likely earned significant pushback from lawmakers, particularly in the wake of an NBC News report that said Sondland had a private discussion with Ukrainian officials at the White House in May, during which he explicitly mentioned Burisma Holdings.

He also told lawmakers he didn't know Hunter Biden was linked to Burisma, despite the fact that it has been widely reported and publicly discussed by Giuliani.

Read more: John Bolton described Rudy Giuliani as a 'hand grenade' that would 'blow everybody up'

Trump loyalists sidelined career officials to further their own political agendas

Sondland has emerged in recent days as one of the central figures in Trump and Giuliani's efforts in Ukraine. And despite his denials, Sondland played an outsize role in the matter, which puzzled US officials because as the ambassador to the EU — which Ukraine isn't a part of — Sondland shouldn't have been involved in the first place.

What George Kent said

On Tuesday, Kent outlined for congressional investigators how he was all but cut out of decisions regarding Ukraine by Sondland, Volker, and Energy Secretary Rick Perry.

Rep. Gerald Connolly, one of the lawmakers who heard Kent's testimony, told The Times that Kent described the move as "wrong."

"Here is a senior State Department official responsible for six countries, one of which is Ukraine, who found himself outside of a parallel process that he felt was undermining 28 years of US policy and promoting the rule of law in Ukraine," Connolly told The Times.

He added that Kent testified that after Volker, Sondland, and Perry sidelined him, they "declared themselves the three people now responsible for Ukraine policy."

What Steve Linick said

Another significant figure in the Ukraine controversy is Linick, the State Department's inspector general.

Linick, who was appointed to his position by Trump, shared with Congress a dossier of conspiracy theories Giuliani transmitted to the State Department earlier this year containing unfounded claims about the Bidens.

Giuliani told CNN he gave the documents to the White House, which then forwarded them to Pompeo. Pompeo gave the documents to a subordinate, who gave them to the department's legal counsel, who ultimately turned them over to Linick.