The ground has shifted on Donald Trump's impeachment - but what happens next?
After more than two years of jousting over Donald Trump's conduct, the ground suddenly shifted in Congress and a move towards impeachment broke free of constraints.
House of Representatives speaker Nancy Pelosi - who for months had resisted Democrat colleagues wanting to impeach the president - launched a formal inquiry, accusing the president of betrayal of his oath of office, national security and the integrity of American elections.
That does not mean the path ahead is all set. Here is a look at what has happened and what might come next:
- Next steps
The House Judiciary Committee will be responsible for recommending articles of impeachment against Mr Trump if the inquiry leads it to do so.
Six House committees have already been investigating alleged impropriety by the president. They will continue to investigate, on an expedited basis, but with no specific deadline.
If the Judiciary panel backs impeachment, the matter goes to the full House for a vote. Democrats control the House and its committees.
If a majority of the House votes for impeachment, it goes to the Senate. It takes a two-thirds vote in the Senate to force a president from office, which would be a daunting challenge for Democrats given Republican control of the upper chamber.
Impeaching a president does not automatically mean his removal. It means the House has voted to bring articles of impeachment and send the process forward. No president has been ousted by impeachment.
- Democrats break their impasse
Some Democrats in Congress have long wanted to kick-start the constitutional process to remove Mr Trump, despite the slim odds of success, but they lacked a critical mass - and Ms Pelosi's support.
An inquiry into the president's machinations to avoid culpability from the Russia investigation came to an indistinct conclusion, with troubling episodes of his behaviour uncovered by special counsel Robert Mueller but no charges recommended for obstructing justice or conspiring with Moscow in its efforts to influence the 2016 US election.
Mr Trump's pre-election payment to a porn actress to maintain her silence and apparent Trump Organisation profiteering from his presidency also fuelled impeachment sentiment from a segment of the party, but it took a whistleblower's still-secret complaint about his dealings with Ukraine to change the landscape.
There's little doubt he pressed Kiev to conduct a corruption investigation of Democratic presidential contender Joe Biden and his son - the president has defiantly stated that he did. He also acknowledged that days before a phone conversation with Ukraine's leader in July, he ordered military aid to Ukraine to be frozen.
The episode raises the possibility that a president used the power of his office to get a foreign government to help him win re-election.
- What has changed?
Ms Pelosi's acceptance of impeachment proceedings is a huge advance for its advocates after proceedings in the Judiciary Committee appeared to be going nowhere. Democrats also believe the focus on Mr Trump's dealings with the Ukrainian leader could resonate more than the Mueller report did.
The Judiciary panel had already begun impeachment hearings and had asked other committees for input. It is not clear that Ms Pelosi's "expedited" timeline will move things along any more quickly. The committee chairman, Democrat Jerrold Nadler, has said he wants to make a decision on whether to recommend articles of impeachment by the end of the year.
- The last time this happened...
In 1998 and 1999, the House - under Republican control - pursued the impeachment of Democratic president Bill Clinton, primarily based on his relationships with women outside his marriage.
The House approved an allegation that Mr Clinton "wilfully provided perjurious, false and misleading testimony" before independent counsel Kenneth Starr's grand jury investigation.
It voted to bring forward the accusation that he "prevented, obstructed and impeded the administration of justice", but the Republican-controlled Senate acquitted him.
- A rarity in history
Only two presidents have been impeached: Andrew Johnson in 1868 and Mr Clinton. Both were acquitted in the Senate.
Richard Nixon, who was the subject of impeachment proceedings, resigned from office in 1974 when it looked certain that the House would impeach him and his prospects in the Senate appeared dire.
- Words from the constitution
The US constitution gives the House "the sole power of impeachment" and the Senate "the sole power to try all impeachments".
It dictates the removal from office of an impeached president who is convicted by the Senate of "treason, bribery or other high crimes and misdemeanours". It is left to Congress to define such terms.