A shock election and pressure campaign ignited Trump impeachment

President Donald Trump's effort to get a foreign government to dig up dirt on his chief Democratic rival started with the surprise election of a new leader in Ukraine in April, touching off a cascade of events that have put his presidency on the line.

As a Democratic-led impeachment inquiry in the House of Representatives heads into public sessions starting Wednesday, Americans will hear directly from key players they've only learned about so far from transcripts, media reports and presidential tweets.

Weeks of testimony have exposed gritty details about how foreign policy is made in the Trump administration. The question now will be whether the president improperly used American might to press Ukraine, a fragile bulwark against Russian aggression in the region, to do his bidding for personal political gain.

Trump has repeatedly called his July 25 conversation with new Ukrainian President Volodymyr Zelenskiy "perfect." Ukrainian officials voiced anxiety that they were getting dragged into a domestic U.S. political fight that would undermine bipartisan support for their nation. Democrats say the president's behavior likely meets the Constitution's definition of "high crimes and misdemeanors."

This week's hearings could lead to Trump becoming just the third American president to be impeached. This is how we got here.

Zelensky's surprise election on April 21 caught U.S. officials off-guard. At the American Embassy in Kyiv and the National Security Council, it fueled a rush by officials to shore up a key ally under siege from Russia. On the Ukraine side, aides quickly tried to secure Trump's participation at Zelenskiy's inauguration as well as an Oval Office handshake.

At the same time, a handful of senior Trump aides, including personal lawyer Rudy Giuliani, seized an opportunity to pressure an inexperienced foreign leader to do what the Trump wanted: Get Ukraine's government to open investigations into former Vice President Joe Biden and his son Hunter as well as pursue unsubstantiated allegations that Ukraine, not Russia, meddled in the 2016 election to help Democrats.

But Giuliani had a problem: he didn't think U.S. Ambassador to Ukraine Marie Yovanovitch was on board with his efforts. A respected career diplomat, Yovanovitch became a casualty of Giuliani's political machinations. Her tenure as envoy ended early, announced by the State Department on May 6 but never officially explained.

That job ended up with Giuliani. In a report published May 14, Giuliani told a Ukrainian outlet that Yovanovitch was removed "because she was part of efforts against the president."

Joining the Giuliani effort to shape Ukraine policy was U.S. Ambassador to the European Union Gordon Sondland, who got his post after donating $1 million to Trump's inaugural committee.

Sondland "was talking about the 2016 elections and an investigation into the Bidens and Burisma," the company on whose board of directors Hunter Biden served, according to testimony from Lieutenant Colonel Alexander Vindman, the Ukraine director on the National Security Council.

As those efforts got underway, in mid-June the Pentagon announced a $250 million aid package for Ukraine, funding that would get held up as Giuliani and Trump sought a public commitment from Zelensky on a "corruption" investigation. An additional $141 million in State Department aid eventually got held hostage as well.

The message to Ukraine was clear: In exchange for meeting the demands detailed by Giuliani, Zelensky would secure a coveted White House meeting and unlock nearly $400 million in security aid he desperately needed to counter Russian forces that had invaded in 2014.

Inside the administration, the widening effort to pressure Ukraine ensnared career diplomats as well as loyal aides such as then-National Security Adviser John Bolton and Secretary of State Michael Pompeo, who found themselves losing control of Ukraine policy -- and powerless to stop it.

Bolton at one point called Giuliani a "hand grenade that is going to blow everyone up," according to Fiona Hill, a former National Security Council senior director. Pompeo "rolled his eyes" when asked about Giuliani's role, Sondland testified. The former New York mayor was just "something we have to deal with," he said.

Ukrainian leaders were left unsure who exactly was in charge. Giuliani and Sondland, who claimed unique access to the president while Pompeo focused on other matters, represented themselves as in control.

"I said, 'Who has said you're 'in charge of Ukraine, Gordon?"' Hill testified of her reaction to Sondland boasting of his leading role. "And he said, 'The president.' Well, that shut me up, because you can't really argue with that. But then I wasn't, to be honest, I wasn't really sure."

Democrats' claims against the president hinge on the July 25 phone call between Trump and Zelensky. A readout of that call, alongside an unnamed whistle-blower's complaint to the intelligence community's inspector general, helped unleash the impeachment inquiry in September.

According to a readout provided by the White House, Zelensky said Ukraine was ready to cooperate further with the U.S. and buy more Javelin anti-tank missiles. "I would like you to do us a favor though," Trump responded -- talking about the need to investigate what role Ukraine may have played in the events that eventually led to Special Counsel Robert Mueller's investigation of Russian interference in the 2016 election.

He followed with a request that Zelensky "look into" allegations that Biden had halted the prosecution of Burisma when he was vice president.

By Aug. 12, concerns about the call by some officials had escalated. A U.S. intelligence official filed a whistle-blower complaint. Four days later, a top Zelensky aide shared with the special representative for Ukraine, Kurt Volker, a draft statement vowing to fight "corruption." Volker said in testimony that he responded by saying the statement didn't contain "Burisma" or "2016 elections." In a separate conversation, Giuliani emphasized that it should.

Reports about the whistle-blower's complaint and a recognition that Ukraine aid was being held up fueled questions from lawmakers and more pushback from officials as the costs of the Giuliani and Trump effort started to spiral wider.

Ambassador William Taylor -- the envoy hand-picked by Pompeo to replace Yovanovitch in May -- sent a cable to Washington on Aug. 29 "describing the folly I saw in withholding military aid to Ukraine at a time when hostilities were still active in the east and when Russia was watching closely to gauge the level of American support for the Ukrainian government."

On Sept. 3, the bipartisan Senate Ukraine Caucus, including Republicans Rob Portman of Ohio and Ron Johnson of Wisconsin, sent a letter to acting Chief of Staff Mick Mulvaney and the Office of Management and Budget, urging the Ukraine aid be released.

On Sept. 10, Bolton departed as national security adviser. It's not clear whether the Ukraine machinations factored into his ouster, but days before he left, he told the State Department to release its portion of the Ukraine aid.

Repeatedly in public and in at least one private conversation, Trump has argued there was no effort to extort anything from Ukraine -- no "quid pro quo." He and his aides have argued instead that Ukraine had long been plagued by corruption, and that the administration's efforts were focused on making sure U.S. assistance wouldn't be wasted.

"The call to the Ukrainian President was PERFECT," Trump tweeted Nov. 10. "Read the Transcript! There was NOTHING said that was in any way wrong. Republicans, don't be led into the fools trap of saying it was not perfect, but is not impeachable."

With his legacy at risk, the public hearings starting this week may also give Trump new ammunition. Already, he has shown no restraint in denigrating people who work for him -- Yovanovitch, he said, was "bad news" and Taylor, her successor, was "human scum." Vindman, according to the president, was a "Never Trumper witness."

Republicans have generally hewed to Trump's defense that Ukraine should investigate corruption, avoiding specific questions about whether the president's references to Biden and the company whose board his son served on swerved the argument into an inappropriate political sphere. But not all.

Condoleezza Rice, a former secretary of State and national security adviser under President George W. Bush, said this week in Abu Dhabi that she found the dueling Ukraine policies "deeply troubling," according to Reuters.

Beyond a small number of Republican lawmakers who have distanced themselves from Trump's call, the president can largely count on support from his political allies in both the House and the Senate.

"Not really anything the president said in that phone call that's different from what he says in public all the time," Rep. Mac Thornberry, a Texas Republican, said Sunday on ABC's "This Week." "Is there some sort of an abuse of power that rises to that threshold that is different than the American people have been hearing for three years? I don't hear that."