Here's what you need to know about the impeachment case against President Trump

US President Donald Trump faces trial in the Senate on two charges, abuse of power and obstruction of Congress.

Both charges stem from allegations that Trump held up military aid to Ukraine to force the government to announce an investigation into his possible 2020 reelection rival, Democrat Joe Biden, and to provide other dirt on the Democrats.

- The Ukraine affair -

The saga began in early 2019 when Trump's personal lawyer Rudy Giuliani began pressuring Ukraine to investigate Burisma Holdings, a Kiev-based energy company on whose board Biden's son Hunter served for five years.

Giuliani also sought an investigation into a debunked conspiracy theory that Ukraine helped the Democrats against Trump in the 2016 election.

According to the allegations, Trump sought both investigations to boost his electoral prospects against Biden.

To boost pressure, Trump canceled Vice President Mike Pence's plan to attend the May 14 inauguration of newly-elected Ukraine President Volodymyr Zelensky.

In a White House meeting on July 10, Gordon Sondland, the US ambassador to the European Union, told senior Ukrainian officials that a high profile summit Zelensky sought with Trump was contingent on the investigations.

Ukraine, a US ally at war in its east following the first Russian invasion of a European nation since the end of the Cold War, relies enormously on Washington's public support.

Nine days later Trump's chief of staff Mick Mulvaney froze a Ukraine military aid package worth $391 million, telling aides it was a Trump directive.

According to the charges, Trump made clear to Zelensky in a July 25 phone call that he wanted the investigations, hinting that they were linked to security assistance and a summit.

Trump mentioned the Bidens and Burisma specifically, and referred to the story that Ukraine helped the Democrats in the 2016 election.

In the ensuing weeks, Sondland pressed Kiev further, saying in text messages that Trump wanted the investigations as a quid pro quo for the military aid.

After Zelensky failed to deliver a statement that US diplomats prepared for him announcing the investigations, on September 1 Sondland told a Zelensky advisor that military aid would not be released until Kiev acted.

By that time an analyst from the US intelligence community had filed a whistleblower report alleging Trump was illegally soliciting foreign interference in a US election.

As that complaint became known, most of the aid was released without Zelensky committing to the investigations.

- Articles of impeachment -

Trump will be tried on two articles of impeachment, or charges, passed by the House of Representatives.

Article I charges Trump with abusing the powers of his office to illicitly solicit Ukraine's interference in the 2020 presidential election.

"President Trump engaged in this scheme or course of conduct for corrupt purposes in pursuit of personal political benefit," the charge says.

"In so doing, President Trump used the powers of the Presidency in a manner that compromised the national security of the United States and undermined the integrity of the United States democratic process."

Article II says that, to protect himself, Trump blocked government agencies and top aides from complying with subpoenas from the House investigation into the Ukraine affair.

"In the history of the Republic, no president has ever ordered the complete defiance of an impeachment inquiry or sought to obstruct and impede so comprehensively the ability of the House of Representatives to investigate 'high crimes and misdemeanors,'" it says.

"This abuse of office served to cover up the president's own repeated misconduct."