A timeline of Trump's campaign to pressure Ukraine's president into investigating Joe Biden

• President Donald Trump and his conservative allies have alleged in recent weeks that former vice president Joe Biden intervened as vice president to force the ouster of a Ukrainian prosecutor to benefit his son Hunter Biden, who was associated with an energy company being investigated for corruption.
• No evidence has emerged to support the claim.
• Here is a timeline on Ukraine's effort to investigate corruption and Trump's campaign to pressure the Ukrainian government to investigate one of his primary political rivals.
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President Donald Trump and his conservative allies have alleged in recent weeks that former vice president Joe Biden intervened as vice president to force the ouster of a Ukrainian prosecutor to benefit his son Hunter Biden, who was associated with an energy company being investigated for corruption. No evidence has emerged to support the claim.

Democrats, for their part, launched an impeachment inquiry into Trump after reports surfaced that he tried to pressure the Ukrainian government into probing the Bidens.

Read more: Trump is facing impeachment over a whistleblower complaint and a phone call with Ukraine's president. Here's what we know.

Here is a timeline of events which include Ukraine's effort to investigate corruption, as well as Trump applying pressure to the Ukrainian government to investigate one of his primary political rivals.

February 2014: Ukrainian president Viktor Yanukovych is ousted in a popular uprising. The new government starts investigating corruption.

• Shortly after assuming power, the new Ukrainian government initiated investigations into corruption throughout the country.

April 2014: After Russian annexation of Crimea, Biden travels to Kiev to support the fledgling pro-Western Ukrainian government.

• The annexation fueled tensions between the American and Russian governments. As a result, Biden began traveling to Ukraine more regularly to show the Obama administration's support for the country as it fends off Russian aggression in the east.

• Biden played a key role coordinating US aid to Ukraine, and also pressured the former Soviet republic to drastically reduce corruption.

May 2014: Hunter Biden is appointed to the board of a Ukrainian energy company, a company under scrutiny for corruption.

• Hunter Biden, then-Vice President Joe Biden' son, joined Burisma Holdings, a natural gas company where he garnered a salary of up to $50,000 a month.

• Burisma had extensive ties to the ousted Yanukovych government, and its owner Mykola Zlochevsky was a focal point of the corruption probe.
• The White House said at the time that Hunter Biden was a private individual, and there were no conflict of interest between him and then-vice president Joe Biden.

December 2015: Biden told Ukrainian leaders to fire the nation's lead prosecutor over his failure to end corruption.

• If Ukrainian Prosecutor General Viktor Shokin wasn't fired, Biden said in Kiev, the country risked losing $1 billion in loan guarantees. Corruption was eating Ukraine "like a cancer," he said.
• Shokin was extensively criticized for failing to curb the power of the Ukrainian elite and slow-walking the corruption probe.

March 29, 2016: The lead Ukrainian prosecutor was fired by parliament.

• The Ukrainian parliament fired Shokin after many Western leaders urged his ouster.
• Yuriy Lutsenko replaced Shokin as the new prosecutor general shortly after.

January 12, 2017: The Ukrainian government formally ended its investigation into Burisma.

• Though the probe ended, Bloomberg reported the investigation had been dormant since a year before then-Vice President Joe Biden started traveling to Ukraine in 2014.

April 2019: Hunter Biden's term as a Burisma board member ends, around the time Joe Biden mounts a third bid for the White House.

• Hunter Biden told The New York Times a month later that he had "no role whatsoever" in the Ukrainian investigation of Burisma or any of its employees.

April 21, 2019: Former comedian Volodymyr Zelensky is elected as the new Ukrainian president.

• Zelensky garnered 73% of the vote, and Trump phoned him to offer congratulations.

Read more: Meet Volodymyr Zelensky, Ukraine's comedian turned president who's at the center of the scandal threatening Trump's presidency

May 2019: Trump's personal lawyer Rudy Giuliani plans - then cancels - a trip to Ukraine to push the government to open probes that could aid Trump politically.

• Giuliani initially defended his trip to The Times, saying, "We're not meddling in an election, we're meddling in an investigation, which we have a right to do."
• On May 11, Giuliani canceled the trip after facing backlash over the appearance of a personal staffer to the president trying to enlist aid from a foreign power to weaken a domestic political rival.
• At some point this month, Giuliani reportedly met with a top Ukrainian prosecutor in Paris. Giuliani told the prosecutor at the time the probe into Burisma should be reopened.

May 16, 2019: Ukraine's top prosecutor said there was no evidence of criminal wrongdoing by either Joe Biden or his son, Hunter Biden.

• "Hunter Biden did not violate any Ukrainian laws - at least as of now, we do not see any wrongdoing," Lutsenko told Bloomberg. "A company can pay however much it wants to its board."

July 18, 2019: Trump delayed nearly $400 million worth of military aid to Ukraine.

• The Trump administration told Congress the aid wasn't granted due to an "interagency delay."

July 25, 2019: Trump spoke with Zelensky over the phone and pressured him into opening a corruption probe on the Bidens.

• Trump asked Zelensky to "do us a favor" and open two investigations, one related to the hacking of the Democratic National Committee during the 2016 election, and another involving the Bidens.
• "There's a lot of talk about Biden's son, that Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the Attorney General would be great. Biden went around bragging that he stopped the prosecution so if you can look into it ... It sounds horrible to me," Trump said, according to a released memo of the call.
• Trump also suggested Attorney General William Barr would play a role in the investigation.

Early August 2019: Giuliani traveled to Spain to meet with a Ukrainian government official. He urged the Zelensky official to investigate Hunter Biden.

• The trip represented a renewed push by Giuliani to dig up dirt that could benefit Trump politically.
• Giuliani portrayed his travels as aimed at helping Ukraine fight corruption. "I can't see how advocating for an investigation of two alleged crimes puts too much pressure on them, other than to do the right thing," he told the Times.

August 12, 2019: A whistleblower within the intelligence community filed a complaint with an internal watchdog over Trump's interaction with Ukraine.

• Intelligence Community Inspector General Michael Atkinson determined the complaint was credible and a matter of "urgent concern."

September 9-10, 2019: The internal watchdog notified Congress of the complaint. Lawmakers demanded to see it.

• House Intelligence Committee Chairman Adam Schiff wrote to Acting Director of National Intelligence Joseph Maguire demanding the complaint be made available to Congress.
• Maguire initially said he wouldn't testify.
• Later, Schiff said Maguire told him he couldn't "because he is being instructed not to, that this involved a higher authority, someone above."

September 11, 2019: The Trump administration released the aid to Ukraine, which had been withheld.

• "The president has made no secret when it comes to foreign assistance that U.S. interests abroad should be prioritized and other foreign countries should also be paying their fair share," a senior Trump administration official told CBS News.

September 23, 2019: Trump suggested the aid to Ukraine was delayed over "corruption" in the country.

• "If you don't talk about corruption, why would you give money to a country that you think is corrupt?" Trump said.

September 24, 2019: Trump acknowledged the funding was delayed, but said other European countries should pay up to support Ukraine's defense.

• "My complaint has always been, and I'd withhold again and I'll continue to withhold until such time as Europe and other nations contribute to Ukraine because they're not doing it," Trump told reporters at the annual United Nations General Assembly.
• Democrats launched impeachment proceedings against Trump.

September 25, 2019: The White House released a rough transcript of Trump's July phone call with Zelensky.

• Trump maintained he did nothing wrong in the phone call.
• "You will see it was a very friendly and totally appropriate call. No pressure and, unlike Joe Biden and his son, NO quid pro quo! This is nothing more than a continuation of the Greatest and most Destructive Witch Hunt of all time!" Trump tweeted.

September 26, 2019: Maguire testified before Congress about the complaint.

• Maguire responded: "I think the whistle-blower did the right thing, he followed the law every step of the way."

• Maguire confirmed that the substance of the whistleblower complaint was "in alignment" with the memo the White House's released on Trump and Zelensky's July call.