In a stark break with diplomatic protocol, President Trump used a cadre of associates to conduct back-channel communications with Ukraine to pressure its government to investigate Democrats, according to witnesses testifying in the impeachment hearings. Here’s what key witnesses say happened:

There was a group that circumvented regular channels for Ukraine policy.

Mr. Trump leaned heavily on his personal lawyer, Rudolph W. Giuliani, and a handful of other officials to carry out his wishes regarding Ukraine.

The back channel Rudolph W. Giuliani Mr. Trump’s personal lawyer Gordon D. Sondland Donor turned E.U. ambassador Kurt D. Volker Former special envoy to Ukraine Rick Perry Energy secretary Mick Mulvaney Acting White House chief of staff The official channel George P. Kent Senior State Department official Marie L. Yovanovitch Former ambassador to Ukraine William B. Taylor Jr. Top American diplomat in Ukraine Lt. Col. Alexander S. Vindman White House Ukraine expert John R. Bolton Former White House national security adviser Fiona Hill Former White House Russia expert Tim Morrison Former senior White House national security aide William B. Taylor Jr., top American diplomat in Ukraine I found a confusing and unusual arrangement for making U.S. policy toward Ukraine. There appeared to be two channels of U.S. policy-making and implementation, one regular and one highly irregular. Oct. 22 opening statement Gordon D. Sondland, donor turned E.U. ambassador Everyone was in the loop. It was no secret. Nov. 20 opening statement Fiona Hill, former White House Russia expert Mr. Giuliani was asserting quite frequently on television in public appearances that he had been given some authority over matters related to Ukraine, and if that was the case, we hadn’t been informed about that. Oct. 14 testimony Mick Mulvaney, acting White House chief of staff You may not like the fact that Giuliani was involved. That’s great. That’s fine. It’s not illegal. It’s not impeachable. The president gets to use who he wants to use. Oct. 17 White House briefing Marie L. Yovanovitch, former ambassador to Ukraine I do not know Mr. Giuliani’s motives for attacking me. But individuals who have been named in the press who have contact with Mr. Giuliani may well have believed that their personal financial ambitions were stymied by our anti-corruption policy in Ukraine. Oct. 11 opening statement George P. Kent, senior State Department official His assertions and allegations against former Ambassador Yovanovitch were without basis, untrue, period. Oct. 15 testimony Marie L. Yovanovitch, former ambassador to Ukraine It sounded like a threat. Nov. 15 testimony According to some witnesses, the group operated outside of the government’s official policy channel, which is made up of national security aides in the White House and diplomats at the State Department. In testimony, some witnesses disputed the idea that there was an “irregular” channel. Gordon D. Sondland, the ambassador to the European Union, pointed to messages and phone calls in which he kept the White House and State Department, including Secretary of State Mike Pompeo, informed of his actions. Many members of the official channel were dismayed that Mr. Giuliani was playing a direct role in policy toward Ukraine. Dr. Hill was in charge of coordinating Ukraine policy across the federal government. Mick Mulvaney, who several witnesses said gave directives related to Ukraine, has defended Mr. Giuliani’s involvement. Beginning in late 2018, Mr. Giuliani and his associates conducted a months-long smear campaign that resulted in the ouster of Marie L. Yovanovitch, a longtime diplomat who had been serving as ambassador to Ukraine. In a July call with the president of Ukraine, Mr. Trump brought up Ms. Yovanovitch, describing her as “bad news,” adding, “She’s going to go through some things.” Ms. Yovanovitch testified that she was “devastated” when she found out.

The main goal of the “back channel” was to push Ukraine to announce investigations into the 2016 election and the Bidens.

Soon after Volodymyr Zelensky was elected president of Ukraine, Mr. Trump appeared to use a highly sought White House visit as leverage.

The back channel The official channel Gordon D. Sondland, donor turned E.U. ambassador Mr. Giuliani demanded that Ukraine make a public statement announcing investigations of the 2016 election/DNC server and Burisma. Mr. Giuliani was expressing the desires of the President of the United States, and we knew that these investigations were important to the President. Nov. 20 opening statement Gordon D. Sondland, donor turned E.U. ambassador Was there a “quid pro quo?” As I testified previously, with regard to the requested White House call and White House meeting, the answer is yes. Nov. 20 opening statement Fiona Hill, former White House Russia expert This is a direct quote from Ambassador Bolton: You go and tell Eisenberg that I am not part of whatever drug deal Sondland and Mulvaney are cooking up on this. Oct. 14 testimony Kurt D. Volker, former special envoy to Ukraine Heard from White House—assuming President Z convinces trump he will investigate / “get to the bottom of what happened” in 2016, we will nail down date for visit to Washington. Text message to Mr. Yermak Lt. Col. Alexander S. Vindman, White House Ukraine expert The parts that were particularly troubling was the references to conducting an investigation. Oct. 29 testimony William B. Taylor Jr., top American diplomat in Ukraine The member of my staff asked Ambassador Sondland what President Trump thought about Ukraine. Ambassador Sondland responded that President Trump cares more about the investigations of Biden. Nov. 13 opening statement Gordon D. Sondland, donor turned E.U. ambassador I recall no discussions with any State Department or White House official about Former Vice President Biden or his son, nor do I recall taking part in any effort to encourage an investigation into the Bidens. Oct. 17 opening statement Kurt D. Volker, former special envoy to Ukraine Had a good chat with Yermak last night. He was pleased with your phone call. Mentioned Z making a statement. Can we all get on the phone to make sure I advise Z correctly as to what he should be saying? Text message to Mr. Giuliani Gordon D. Sondland, donor turned E.U. ambassador Do we still want Ze to give us an unequivocal draft with 2016 and Boresma? Text message to Mr. Volker Kurt D. Volker, former special envoy to Ukraine That’s the clear message so far ... Text message to Mr. Sondland Kurt D. Volker, former special envoy to Ukraine At no time was I aware of or took part in an effort to urge Ukraine to investigate former Vice President Biden. Oct. 3 opening statement Fiona Hill, former White House Russia expert It is not credible to me at all that he was oblivious. Nov. 21 testimony In a meeting after Mr. Zelensky’s inauguration, Mr. Trump directed three officials, whom some referred to as “the three amigos,” to work through Mr. Giuliani about his “concerns” related to Ukraine. Burisma is a Ukrainian gas company that hired Hunter Biden, the son of former Vice President Joseph R. Biden Jr., to serve on its board. Dr. Hill testified that Mr. Sondland was “involved in a domestic political errand,” diverging from the regular Ukraine policy. John R. Bolton, then Mr. Trump’s national security adviser, was furious when he found out on July 10 that Mr. Trump’s meeting with Mr. Zelensky was being predicated on the Ukrainian president announcing investigations, according to Dr. Hill’s testimony. Mr. Bolton, who is waiting for a judge to rule on whether he should testify, was referring to John A. Eisenberg, the chief legal adviser for the National Security Council. On the morning of the call at the center of the whistle-blower complaint, Mr. Volker texted one of Mr. Zelensky’s top advisers, Andrey Yermak. During the July 25 call, Mr. Trump asked Mr. Zelensky to “do us a favor” and “find out what happened.” Two White House officials who listened in on the call were concerned by what they had heard. Mr. Trump has repeatedly said he did nothing wrong and that his call with Mr. Zelensky was “perfect.” The day after the phone call, an aide to Mr. Taylor overheard a telephone conversation in Kyiv between Mr. Trump and Mr. Sondland in which they appeared to discuss the investigations. Mr. Sondland did not mention the conversation in his original testimony, but he later confirmed that it had occurred. Text messages written by Mr. Volker and Mr. Sondland over the summer show that the two men attempted to get the Ukrainian president to make a statement about the investigations. In testimony, Mr. Volker later claimed he did not realize Burisma was connected to the Biden family. Mr. Sondland testified that until as late as September, he also did not realize that Burisma was linked to the Bidens. Other witnesses said that they had understood in the spring and summer that Burisma was “code for the Bidens.”

Trump was holding back critical aid to Ukraine that had been approved by Congress.

National security officials learned in a July meeting that Mr. Trump had directed Mr. Mulvaney to hold up $391 million in aid to Ukraine. The meeting was described in the whistle-blower complaint and corroborated by several witnesses.

The back channel The official channel William B. Taylor Jr., top American diplomat in Ukraine In an instant, I realized that one of the key pillars of our strong support for Ukraine was threatened. The irregular policy channel was running contrary to the goals of longstanding U.S. policy. Oct. 22 opening statement Lt. Col. Alexander S. Vindman, White House Ukraine expert I don’t think there was much clarity as to why it was being held up. Oct. 29 opening statement William B. Taylor Jr., top American diplomat in Ukraine Over 13,000 Ukrainians had been killed in the war, one or two a week. More Ukrainians would undoubtedly die without the U.S. assistance. Oct. 22 opening statement The congressionally approved aid was viewed as critical to support Ukraine’s defenses against Russian-backed forces.

The release of the aid was contingent on a public announcement of investigations, according to witnesses.

Ukraine officials may have been aware that security aid was cut off by July 25, much earlier than previously known, according to testimony by a top Pentagon official. In an addendum, Mr. Sondland directly contradicted his earlier testimony and admitted to House investigators that he told Mr. Yermak how the aid could be restored.