WASHINGTON _ A whistleblower, a phone call and critical military aid are what set the House Democrats' formal impeachment inquiry into President Donald Trump in motion this week.
The investigation is focused on a conversation between Trump and Ukrainian President Volodymyr Zelenskiy in which Trump pressed Zelenskiy to investigate former U.S. Vice President Joe Biden, Trump's possible opponent in the 2020 presidential election. Trump has acknowledged that at the same time he was withholding aid to the country.
Here's a look at some of the main events that led up to the impeachment inquiry and what has happened since the announcement.
_ 2014: The United States becomes one of Ukraine's strongest allies.
President Barack Obama's administration begins funneling millions of dollars in aid to Ukraine after the country's president, Viktor Yanukovich, was ousted in a 2014 revolution for backing separatists who supported Russia's annexation of Ukraine's Crimean peninsula. With a new but fragile pro-Western government in power, Vice President Biden oversees the effort in which U.S. funds are aimed at boosting Ukrainian armed forces in the face of Russia's military involvement.
Months after Biden assumed the role, his son Hunter Biden had joined the board of Burisma Holdings, a private Ukrainian natural gas company _ a position that has since raised some concerns about a potential conflict of interest. It was during this period that Burisma's owner was being investigated by Ukrainian prosecutors over possible financial abuses, although Hunter Biden was not accused of any wrongdoing.
White House officials pointed to Hunter Biden's status as a private citizen, and the vice president said his son made his own business decisions.
_ Late 2015: Obama administration and its European allies pressure Ukraine to remove top prosecutor.
While Vice President Biden represents U.S. interests in Ukraine, the Obama administration and its European allies push to remove the country's top prosecutor, Viktor Shokin, as part of a crackdown on corruption _ a long-standing problem in the country. As part of the maneuver, Biden threatens to withhold a $1 billion loan guarantee to Ukraine if Shokin doesn't resign.
Shokin had led the investigation into the owner of Burisma, although the inquiry was dormant at the time Biden pushed for the prosecutor's ousting, The Washington Post reported.
No evidence indicated Biden or his son acted improperly.
_ 2016: Shokin is out.
In March of 2016, Ukrainian officials vote to oust Shokin. The same year, a Kyiv district court finds no evidence of criminal wrongdoing by Burisma's owner, according to CNBC.
_ April 2019: Zelensky elected president of Ukraine.
Hunter Biden's term on the Burisma board expires and he steps down from the company, according to Reuters.
The same month, comedian Volodymyr Zelenskiy is elected Ukraine's leader. He pledges to focus on ending the war against Russia-backed separatists in the country's east.
_ May 2019: Rudolph W. Giuliani skips Ukraine trip after Democrats say he tried to recruit the country to help sway 2020 election.
Trump's personal lawyer cancels a visit to Ukraine after Democrats denounce his effort to push the country to open investigations that he hoped would benefit Trump politically. Democrats say the plan signaled a clear attempt to recruit a foreign nation to influence a U.S. election.
"We're not meddling in an election, we're meddling in an investigation, which we have a right to do," Giuliani said in an interview with The New York Times.
_ July 25: The phone call.
Trump directly asks Ukrainian President Zelenskiy for a "favor" while discussing U.S. military aid during a July 25 phone call, according to a White House memo that would later be released.
The phone call was one day after former special counsel Robert S. Mueller III testified before Congress on the nearly two-year investigation he led into Russian election interference in 2016 and potential obstruction of justice.
_ Aug. 12: Complaint filed.
The whistleblower complaint is filed with the intelligence community's inspector general, Michael Atkinson.
_ Sept. 9: House Intelligence Committee notified of complaint.
Atkinson notifies the House Intelligence Committee about the complaint, which he characterizes as "urgent" and "credible."
_ Sept. 13: Intelligence Committee chairman issues subpoena for complaint.
Intelligence Committee Chairman Rep. Adam Schiff, D-Calif., issues a subpoena for Joseph Maguire, the acting director of national intelligence, to appear before the panel after Schiff said Maguire had not transmitted the complaint to Congress within seven days, "in violation of the law."
_ Sept. 18: Reports emerge that Trump made a "promise" to a foreign leader.
The Washington Post reports that the whistleblower complaint is about a conversation between Trump and a foreign leader.
_ Sept. 19: Intelligence inspector general refuses to tell Congress about whistleblower's complaint.
The government's intelligence inspector general refuses to discuss the substance of the whistleblower complaint at a closed-door House Intelligence Committee meeting. Trump rejects the report as "fake news" and "presidential harassment." Some of the whistleblower's allegations appear to center on Ukraine, according to reports from The Washington Post and The New York Times.
_ Sept. 20: Ukraine scandal emerges.
Trump defends himself as House Democrats demand the release of the whistleblower complaint. The controversy refocuses attention on Trump's attempts to undercut Biden after reports that the president urged Ukrainian officials to investigate Hunter Biden's business dealings for possible corruption.
_ Tuesday: House Democrats launch an impeachment inquiry.
Speaker Nancy Pelosi, D-Calif., announces that the House will begin a formal impeachment inquiry of the president.
"The actions of the Trump presidency have revealed the dishonorable fact of the president's betrayal of his oath of office, betrayal of our national security and betrayal of the integrity of our elections," Pelosi says. "No one is above the law."
_ Wednesday: The memo is released.
The White House releases its account of the 30-minute Trump-Zelenskiy call.
According to the memo, Trump asked Zelenskiy both to investigate Biden and to look into CrowdStrike, a cybersecurity firm that did work for the Democrats in the 2016 election. Trump asked Zelenskiy at least five separate times on the call to work with Attorney General William Barr on such investigations.
_ Thursday: The complaint is released and Maguire testifies.
The House Intelligence Committee releases a redacted version of the complaint in which a whistleblower alleges that White House officials took unusual steps to secure the transcript of Trump's July 25 call with Zelensky. In the complaint, the whistleblower told the intelligence community's inspector general that Trump was "using the power of his office to solicit interference from a foreign country in the 2020 U.S. election."
Pelosi characterizes the whistleblower's account as evidence of a White House "cover-up." She says the allegations about Trump's contacts with Ukraine would now be the central focus of the Democrats' impeachment investigation.
At a contentious House committee hearing Thursday morning, Maguire, the acting director of national intelligence, is criticized by Democrats for initially failing to disclose the whistleblower's complaint.
The Los Angeles Times reports that, meanwhile, Trump raged at the Democrats' new impeachment proceedings and slammed the intelligence officer and the White House aides who helped the whistleblower as "almost a spy," and suggested it was treason.
