The Democrats have announced an impeachment inquiry into Donald Trump, sparked by a whistleblower complaint alleging the U.S. president sought dirt on a political rival from a foreign government. 

The whistleblower's main allegation was that Trump pressured Ukrainian President Volodymyr Zelensky to pursue an investigation related to Joe Biden, a leading contender to be the Democratic nominee in the 2020 presidential election, while withholding military aid money as a form of leverage. 

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What are the Trump whistleblower's key allegations?

Trump has stuck to a theory perpetuated by his personal lawyer, Rudy Giuliani, that Biden acted to benefit his son Hunter – but no evidence that either Biden or his son acted improperly has been found. 

Here's a timeline of the key events that led to the claims about Biden, the infamous Trump-Zelensky call, and what happened next:

2014

April 21: Joe Biden, then vice president, visits Kiev in a show of support for Ukraine's government.

May 13: Hunter Biden, son of Joe Biden, is appointed to the board of Ukrainian natural gas company Burisma Holdings. 

The company was already under investigation by Ukrainian authorities when Biden was appointed on a 50,000 U.S. dollars/month salary, and its owner Mykola Zlochevsky was being investigated by Britain's Serious Fraud Office.

2015

December: Joe Biden, visiting Ukraine, hits out at corruption and warns the country could lose one billion U.S. dollars in loan guarantees if prosecutor general Viktor Shokin isn't fired. Other Western countries also call for Shokin to go, arguing that he is not doing enough to crack down on corruption. 

Ukrainian officials told Bloomberg Shokin's inquiry was already dormant when the vice president issued the warning.

2016

March: Shokin is removed following a vote by the Ukrainian parliament in a decision welcomed by the European Union.

2017

January: Ukraine formally ends its investigation into Burisma.

2018

Late: Giuliani speaks to Shokin by Skype and contacts other former Ukrainian prosecutors.

2019

January: Giuliani meets then Ukrainian prosecutor general Yuriy Lutsenko.

April: Hunter Biden leaves the Burisma board and Volodymyr Zelensky is elected Ukrainian president.

May 7: Trump recalls U.S. Ambassador to Ukraine Marie Yovanovitch from her posting in Kiev. 

May 11: Giuliani cancels a planned trip to Ukraine after his plan to push the country's new leadership to launch investigations that could help Trump was criticized. 

July 18: Trump delays up to 400 million U.S. dollars in military aid to Ukraine.

July 24: Special Counsel Robert Mueller testifies before Congress.

July 25: Trump and Zelensky discuss a "favor" in the now infamous phone call that sparked the whistleblower complaint.

July 28: Director of National Intelligence (DNI) Dan Coats is sacked by Trump.

Early August: Giuliani meets a Ukrainian official in Madrid.

August 12: The whistleblower, who remain anonymous, issues a complaint to Intelligence Community Inspector General (ICIG) Michael Atkinson.

September 24: House Speaker Nancy Pelosi announces that she will launch an impeachment inquiry into Trump.

September 25: A summary of the Trump-Zelensky call is released.

September 26: The whistleblower's complaint is released.

September 27: Kurt Volker, U.S. special envoy for Ukraine, resigns. 

October 3: Volker testifies behind closed doors, providing Congress with a text message chain documenting diplomats' attempts to persuade Ukraine to launch investigations requested by Trump.

October 6: Reports emerge that a second whistleblower with information about Trump's dealings with Ukraine has come forward. 

October 8: The White House announces it will not cooperate with the impeachment inquiry.