Like every presidential conversation with a foreign leader, this one had scripted talking points and a predigested news release recounting an exchange yet to take place.

Aides in the White House situation room clustered around a speaker phone, pens and pads in hand to document what they heard.

At 9:03am on Thursday 25 July, they listened as Donald Trump picked up the phone in the White House residence and was connected to Volodymr Zelensky, the newly elected president of Ukraine. Within minutes, two note-takers exchanged troubled looks.

Trump had not merely veered off his talking points. By the conversation’s end, he had asked Zelensky — a leader in dire need of US military aid to fight the Russian-led invasion on his eastern border — to “do us a favour” by investigating one of his political rivals and an unfounded conspiracy theory about the 2016 election.

That 30-minute conversation has now emerged as a mortal threat to Trump’s presidency. This week, the House of Representatives begins public hearings that could lead to the impeachment of a president for only the third time in US history.

Trump impeachment: Who's who in the Ukraine scandal Show all 26 1 /26 Trump impeachment: Who's who in the Ukraine scandal Trump impeachment: Who's who in the Ukraine scandal Donald Trump Accused of abusing his office by pressing the Ukrainian president in a July phone call to help dig up dirt on Joe Biden, who may be his Democratic rival in the 2020 election. He also believes that Hillary Clinton’s deleted emails - a key factor in the 2016 election - may be in Ukraine, although it is not clear why. EPA Trump impeachment: Who's who in the Ukraine scandal The Whistleblower Believed to be a CIA agent who spent time at the White House, his complaint was largely based on second and third-hand accounts from worried White House staff. Although this is not unusual for such complaints, Trump and his supporters have seized on it to imply that his information is not reliable. Expected to give evidence to Congress voluntarily and in secret. Getty Trump impeachment: Who's who in the Ukraine scandal The Second Whistleblower The lawyer for the first intelligence whistleblower is also representing a second whistleblower regarding the President's actions. Attorney Mark Zaid said that he and other lawyers on his team are now representing the second person, who is said to work in the intelligence community and has first-hand knowledge that supports claims made by the first whistleblower and has spoken to the intelligence community's inspector general. The second whistleblower has not yet filed their own complaint, but does not need to to be considered an official whistleblower. Getty Trump impeachment: Who's who in the Ukraine scandal Rudy Giuliani Former mayor of New York, whose management of the aftermath of the September 11 attacks in 2001 won him worldwide praise. As Trump’s personal attorney he has been trying to find compromising material about the president’s enemies in Ukraine in what some have termed a “shadow” foreign policy. In a series of eccentric TV appearances he has claimed that the US state department asked him to get involved. Giuliani insists that he is fighting corruption on Trump’s behalf and has called himself a “hero”. AP Trump impeachment: Who's who in the Ukraine scandal Volodymyr Zelensky The newly elected Ukrainian president - a former comic actor best known for playing a man who becomes president by accident - is seen frantically agreeing with Trump in the partial transcript of their July phone call released by the White House. With a Russian-backed insurgency in the east of his country, and the Crimea region seized by Vladimir Putin in 2014, Zelensky will have been eager to please his American counterpart, who had suspended vital military aid before their phone conversation. He says there was no pressure on him from Trump to do him the “favour” he was asked for. Zelensky appeared at an awkward press conference with Trump in New York during the United Nations general assembly, looking particularly uncomfortable when the American suggested he take part in talks with Putin. AFP/Getty Trump impeachment: Who's who in the Ukraine scandal Mike Pence The vice-president was not on the controversial July call to the Ukrainian president but did get a read-out later. However, Trump announced that Pence had had “one or two” phone conversations of a similar nature, dragging him into the crisis. Pence himself denies any knowledge of any wrongdoing and has insisted that there is no issue with Trump’s actions. It has been speculated that Trump involved Pence as an insurance policy - if both are removed from power the presidency would go to Democratic House Speaker Nancy Pelosi, something no Republican would allow. AP Trump impeachment: Who's who in the Ukraine scandal Rick Perry Trump reportedly told a meeting of Republicans that he made the controversial call to the Ukrainian president at the urging of his own energy secretary, Rick Perry, and that he didn’t even want to. The president apparently said that Perry wanted him to talk about liquefied natural gas - although there is no mention of it in the partial transcript of the phone call released by the White House. It is thought that Perry will step down from his role at the end of the year. Getty Trump impeachment: Who's who in the Ukraine scandal Joe Biden The former vice-president is one of the frontrunners to win the Democratic nomination, which would make him Trump’s opponent in the 2020 election. Trump says that Biden pressured Ukraine to sack a prosecutor who was investigating an energy company that Biden’s son Hunter was on the board of, refusing to release US aid until this was done. However, pressure to fire the prosecutor came on a wide front from western countries. It is also believed that the investigation into the company, Burisma, had long been dormant. Reuters Trump impeachment: Who's who in the Ukraine scandal Hunter Biden Joe Biden’s son has been accused of corruption by the president because of his business dealings in Ukraine and China. However, Trump has yet to produce any evidence of corruption and Biden’s lawyer insists he has done nothing wrong. AP Trump impeachment: Who's who in the Ukraine scandal William Barr The attorney-general, who proved his loyalty to Trump with his handling of the Mueller report, was mentioned in the Ukraine call as someone president Volodymyr Zelensky should talk to about following up Trump’s preoccupations with the Biden’s and the Clinton emails. Nancy Pelosi has accused Barr of being part of a “cover-up of a cover-up”. AP Trump impeachment: Who's who in the Ukraine scandal Mike Pompeo The secretary of state initially implied he knew little about the Ukraine phone call - but it later emerged that he was listening in at the time. He has since suggested that asking foreign leaders for favours is simply how international politics works. Gordon Sondland testified that Pompeo was "in the loop" and knew what was happening in Ukraine. Pompeo has been criticised for not standing up for diplomats under his command when they were publicly criticised by the president. AFP via Getty Trump impeachment: Who's who in the Ukraine scandal Nancy Pelosi The Democratic Speaker of the House had long resisted calls from within her own party to back a formal impeachment process against the president, apparently fearing a backlash from voters. On September 24, amid reports of the Ukraine call and the day before the White House released a partial transcript of it, she relented and announced an inquiry, saying: “The president must be held accountable. No one is above the law.” Getty Trump impeachment: Who's who in the Ukraine scandal Adam Schiff Democratic chairman of the House intelligence committee, one of the three committees leading the inquiry. He was criticized by Republicans for giving what he called a “parody” of the Ukraine phone call during a hearing, with Trump and others saying he had been pretending that his damning characterisation was a verbatim reading of the phone call. He has also been criticised for claiming that his committee had had no contact with the whistleblower, only for it to emerge that the intelligence agent had contacted a staff member on the committee for guidance before filing the complaint. The Washington Post awarded Schiff a “four Pinocchios” rating, its worst rating for a dishonest statement. Reuters Trump impeachment: Who's who in the Ukraine scandal Lev Parnas and Igor Fruman Florida-based businessmen and Republican donors Lev Parnas (pictured with Rudy Giuliani) and Igor Fruman were arrested on suspicion of campaign finance violations at Dulles International Airport near Washington DC on 9 October. Separately the Associated Press has reported that they were both involved in efforts to replace the management of Ukraine's gas company, Naftogaz, with new bosses who would steer lucrative contracts towards companies controlled by Trump allies. There is no suggestion of any criminal activity in these efforts. Reuters Trump impeachment: Who's who in the Ukraine scandal William Taylor The most senior US diplomat in Ukraine and the former ambassador there. As one of the first two witnesses in the public impeachment hearings, Taylor dropped an early bombshell by revealing that one of his staff – later identified as diplomat David Holmes – overheard a phone conversation in which Donald Trump could be heard asking about “investigations” the very day after asking the Ukrainian president to investigate his political enemies. Taylor expressed his concern at reported plans to withhold US aid in return for political smears against Trump’s opponents, saying: “It's one thing to try to leverage a meeting in the White House. It's another thing, I thought, to leverage security assistance -- security assistance to a country at war, dependent on both the security assistance and the demonstration of support." Getty Images Trump impeachment: Who's who in the Ukraine scandal George Kent A state department official who appeared alongside William Taylor wearing a bow tie that was later mocked by the president. He accused Rudy Giuliani, Mr Trump’s personal lawyer, of leading a “campaign of lies” against Marie Yovanovitch, who was forced out of her job as US ambassador to Ukraine for apparently standing in the way of efforts to smear Democrats. Getty Images Trump impeachment: Who's who in the Ukraine scandal Marie Yovanovitch One of the most striking witnesses to give evidence at the public hearings, the former US ambassador to Ukraine received a rare round of applause as she left the committee room after testifying. Canadian-born Yovanovitch was attacked on Twitter by Donald Trump while she was actually testifying, giving Democrats the chance to ask her to respond. She said she found the attack “very intimidating”. Trump had already threatened her in his 25 July phone call to the Ukrainian president saying: “She’s going to go through some things.” Yovanovitch said she was “shocked, appalled and devastated” by the threat and by the way she was forced out of her job without explanation. REUTERS Trump impeachment: Who's who in the Ukraine scandal Alexander Vindman A decorated Iraq War veteran and an immigrant from the former Soviet Union, Lt Col Vindman began his evidence with an eye-catching statement about the freedoms America afforded him and his family to speak truth to power without fear of punishment. One of the few witnesses to have actually listened to Trump’s 25 July call with the Ukrainian president, he said he found the conversation so inappropriate that he was compelled to report it to the White House counsel. Trump later mocked him for wearing his military uniform and insisting on being addressed by his rank. Getty Images Trump impeachment: Who's who in the Ukraine scandal Jennifer Williams A state department official acting as a Russia expert for vice-president Mike Pence, Ms Williams also listened in on the 25 July phone call. She testified that she found it “unusual” because it focused on domestic politics in terms of Trump asking a foreign leader to investigate his political opponents. Getty Images Trump impeachment: Who's who in the Ukraine scandal Kurt Volker The former special envoy to Ukraine was one of the few people giving evidence who was on the Republican witness list although what he had to say may not have been too helpful to their cause. He dismissed the idea that Joe Biden had done anything corrupt, a theory spun without evidence by the president and his allies. He said that he thought the US should be supporting Ukraine’s reforms and that the scheme to find dirt on Democrats did not serve the national interest. Getty Images Trump impeachment: Who's who in the Ukraine scandal Tim Morrison An expert on the National Security Council and another witness on the Republican list. He testified that he did not think the president had done anything illegal but admitted that he feared it would create a political storm if it became public. He said he believed the moving the record of the controversial 25 July phone call to a top security server had been an innocent mistake. Getty Images Trump impeachment: Who's who in the Ukraine scandal Gordon Sondland In explosive testimony, one of the men at the centre of the scandal got right to the point in his opening testimony: “Was there a quid pro quo? Yes,” said the US ambassador to the EU who was a prime mover in efforts in Ukraine to link the release of military aid with investigations into the president’s political opponents. He said that everyone knew what was going on, implicating vice-president Mike Pence and secretary of state Mike Pompeo. The effect of his evidence is perhaps best illustrated by the reaction of Mr Trump who went from calling Sondland a “great American” a few weeks earlier to claiming that he barely knew him. AP Trump impeachment: Who's who in the Ukraine scandal Laura Cooper A Pentagon official, Cooper said Ukrainian officials knew that US aid was being withheld before it became public knowledge in August – undermining a Republican argument that there can’t have been a quid pro quo between aid and investigations if the Ukrainians didn’t know that aid was being withheld. Getty Images Trump impeachment: Who's who in the Ukraine scandal David Hale The third most senior official at the state department. Hale testified about the treatment of Marie Yovanovitch and the smear campaign that culminated in her being recalled from her posting as US ambassador to Ukraine. He said: “I believe that she should have been able to stay at post and continue to do the outstanding work.” EPA Trump impeachment: Who's who in the Ukraine scandal Fiona Hill Arguably the most confident and self-possessed of the witnesses in the public hearings phase, the Durham-born former NSC Russia expert began by warning Republicans not to keep repeating Kremlin-backed conspiracy theories. In a distinctive northeastern English accent, Dr Hill went on to describe how she had argued with Gordon Sondland about his interference in Ukraine matters until she realised that while she and her colleagues were focused on national security, Sondland was “being involved in a domestic political errand”. She said: “I did say to him, ‘Ambassador Sondland, Gordon, this is going to blow up’. And here we are.” AP Trump impeachment: Who's who in the Ukraine scandal David Holmes The Ukraine-based diplomat described being in a restaurant in Kiev with Gordon Sondland while the latter phoned Donald Trump. Holmes said he could hear the president on the other end of the line – because his voice was so “loud and distinctive” and because Sondland had to hold the phone away from his ear – asking about the “investigations” and whether the Ukrainian president would cooperate. REUTERS

More than half a dozen Trump administration officials have called the phone conversation and the events surrounding it insidious and shocking. Five officials who dealt with Ukraine have resigned since September.

The unfolding story is in many ways a sequel to the events that led to Robert Mueller’s investigation into Russian interference in the 2016 election. Once again, the plot involves foreign influence in an election and is centred in the post-Soviet sphere.

Only one day before Trump spoke to Zelensky, Mueller had testified to Congress about how the Russians had tried to help elect Trump by organising the theft and release of emails damaging to his opponent. In that case, the Russians were the pursuers who sought contacts with Trump’s campaign.

Now the president and his minions were the aggressors, seeking help with the 2020 re-election effort.

They asked the Ukrainians to investigate unfounded allegations about former vice president Joe Biden, one of Trump’s leading Democratic rivals, as well as to chase a conspiracy theory that Ukraine, not Russia, had intervened in 2016.

The story is also another chapter in Trump’s war on the wheels of US governance, from the intelligence community to the diplomatic corps to Congress itself.

In his zeal to win Zelensky’s compliance, the president ousted the US ambassador to Ukraine, froze congressionally approved military aid, shut out foreign-policy experts in the National Security Council and sidestepped the State Department to set up a back-channel to Kiev with his personal lawyer, Rudy Giuliani.

The Ukraine saga is yet another episode in which Russia is the potential beneficiary of White House decisions.

Trump not only sought to muddy the picture of Russia’s role in the 2016 election, but also withheld nearly $400m (£311m) in military aid, a tenth of Ukraine’s defence budget, for its war with Russian-backed forces.

The Russians “would love the humiliation of Zelensky at the hands of the Americans”, William B Taylor Jr, the top diplomat in Kiev who nearly quit in protest, testified to Congress.

This account of the effort to muscle the Ukrainians for Trump’s political gain is based on interviews with more than a dozen American and Ukrainian principals as well as thousands of pages of witnesses’ testimony in the House impeachment inquiry.

More details and revelations are likely to surface in the hearings that begin on Wednesday in the historic House ways and means committee room on Capitol Hill.

Trump's Ukraine call not a problem because he speaks like that 'all the time', says Rep Mac Thornberry

Zelensky’s election in April thrilled American foreign policy experts who had watched Ukraine struggle for decades in the shadow of Russian economic and military threats, seesawing between democracy and authoritarianism.

Zelensky hoped to cement a relationship with the US president. But even before he took office, his aides suspected that the route to Trump ran through Giuliani rather than the State Department or the National Security Council.

The former New York mayor’s influence over administration policy towards Ukraine “was almost unmissable”, George Kent, a deputy assistant secretary of state, testified.

Giuliani and his allies had worked for months to force out Marie Yovanovitch, the US ambassador in Kiev, claiming, with no evidence, that she was disloyal to Trump.

Yovanovitch, whose superiors insisted that she was an exemplary public servant who had been falsely accused, testified that she did not know exactly why, but she believed that Giuliani and Trump saw her as an obstacle to their strategy for Ukraine.

Even before she was ousted, Giuliani and the president were levelling charges on Fox News against one of Trump’s main Democratic rivals, Biden.

As vice president, they claimed, Biden had forced Zelensky’s predecessor to fire a state prosecutor to quash an investigation of a Ukrainian gas company, Burisma, that had hired Biden’s son Hunter.

No evidence has emerged to support that charge or that points to any crime by Hunter Biden.

Trump had also embraced a fringe theory, debunked by extensive evidence, that a central event in the 2016 campaign – the theft of private emails from Democratic computers and their release on the rogue website WikiLeaks – had not been carried out by Russia, as both US intelligence experts and a criminal inquiry had proved, but by Ukraine.

Three days after Zelensky’s swearing-in on 20 May, Gordon Sondland, a Republican donor with no diplomatic experience whom Trump had appointed ambassador to the European Union, and Kurt Volker, a US special envoy to Ukraine, went to an Oval Office meeting. There they praised Zelensky as a reformer who deserved US support.

The president would have none of it. “They are all corrupt, they are all terrible people,” Trump retorted, according to Volker. He added: “They tried to take me down.”

Trump was apparently referring to Ukraine’s disclosure in 2016 of tens of millions of dollars in secret payments to Paul Manafort, Trump’s campaign chairman at the time, who had worked as a consultant to one of Zelensky’s predecessors.

Former US ambassador to Ukraine Marie Yovanovitch arrives on Capitol Hill to testify in the impeachment inquiry (APAP Photo/J Scott Applewhite)

Manafort was forced to resign from the Trump campaign and is now in prison for crimes related to those payments.

In the Oval Office, the president told both men to coordinate future Ukraine-related initiatives with Giuliani. “He just kept saying: ‘Talk to Rudy, talk to Rudy,’” Sondland testified.

And they did, creating a foreign policy back-channel that bewildered both Ukrainians and high-ranking administration officials.

Among those officials was John Bolton, Trump’s third national security adviser. Just days earlier, he had warned Fiona Hill, one of his top deputies: “Giuliani is a hand grenade who’s going to blow everybody up.”

On 18 July, a secure video conference call with national security officials was interrupted by the disembodied voice of an Office of Management and Budget staffer.

At the direction of Mick Mulvaney, the president’s acting chief of staff, the staffer said, the office had placed a hold on $391m (£304m) in military aid for Ukraine. Taylor, the top US envoy to Ukraine, said he listened “in astonishment”.

It was against this backdrop that Trump’s conversation with Zelensky on 25 July unfolded. The president told Zelensky that the United States had done much for his nation and raised the “favour” he wanted – the inquiries into the 2016 election and the Bidens.

What the Bidens had done “sounds horrible to me”, he said, according to a White House reconstructed transcript of the call. Trump added that Giuliani would be in touch. Zelensky assured Trump that a new prosecutor would investigate Burisma.

But Alexander Vindman, Bolton’s chief Ukraine specialist, who was taking notes on the conversation in the situation room, was staggered by the implications of Trump’s remarks.

He headed for the office of John Eisenberg, the National Security Council’s chief legal adviser, to question the propriety of the demand for investigations.

Eisenberg quickly shunted the official summary of the conversation to an electronic storage system normally used for the most sensitive classified information. Later, he instructed Vindman not to discuss the phone call with others.

Nonetheless, a CIA officer detailed to the White House got wind of it. On 12 August, he filed a whistleblower complaint. Word of the whistleblower complaint had reached the top echelons of the National Security Council. As soon as Congress learned of it in early September, three House committees opened investigations.

Adam Schiff, chairman of the House Intelligence Committee (AP Photo/J Scott Applewhite)

Volker called the transcript of the 25 July call “explosive” and resigned just before he testified.

Kent, the deputy assistant secretary of state, told investigators that Trump had stood US policy on its head. For decades, he said, the United States had demanded that leaders in Ukraine and other countries stop instigating politically motivated prosecutions of their opponents and uphold the rule of law.

For Trump to ask Ukraine to investigate his political rival for his political gain, he said, was “wrong”.

Hill testified that she was “shocked” and “very saddened” by the transcript of the calls and documents. Together, she said, they confirmed “my worst fears and nightmares” that private interests had subverted America’s national security concerns.

Her former boss, Bolton, who resigned in September, has said he would not testify unless a federal court rules that he can legally do so.

But in a letter to the court last week, his lawyer suggested more is to come. Bolton, he said, knows of many other White House meetings and discussions about Ukraine that have yet to become public.