Donald Trump’s second impeachment trial is underway in the US - with the former president the first in history to be impeached twice. 

The proceedings surrounding the impeachment have been controversial since before the House met, with Trump's defence team argued that he could not face trial after leaving the White House.

Despite the claims, a  56-44 majority voted in favour of continuing, with a handful of Republicans backing the trial.

So with the vote likely to dominate US politics, we take a look at everything you need to know.

HeraldScotland:

What has Donald Trump been charged with?

The decision to impeach Trump is unprecedented given that he is no longer in the White House. 

The House impeached Trump on January 13th  with a single charge of  "incitement of insurrection" for allegedly encouraging the riot at the U.S. Capitol, which saw 5  people die. 

Thousands gathered to show support for Trump after he claimed that his loss in the 2020 election was a result of widespread electoral fraud. 

HeraldScotland:

What is impeachment?

Impeachment is the process which allows Congress to put presidents on trial, however, it is a political process rather than a criminal trial. 

Articles of impeachment are charges brought against a president by the House of Representatives. If the House votes to pass them, proceedings move to the Senate, which decides whether or not to convict.

Mr Trump was the third US president to have been impeached, with Richard Nixon famously resigning before he could be impeached. 

The two others, Bill Clinton in 1999 and Andrew Johnson in 1868, were left in office by the Senate.

What is the defence of Donald Trump?

The defence of Donald Trump has taken a bit of a blow already with senators voting that the trial should go ahead, with the main defence being that such a proceeding was unconstitutional. The lawyers of Donald Trump insist that the president should not be tried as he is no longer in office. In a sign of what is likely to come, 45 Republicans signed a motion ahead of trial stating that the trial itself was unconstitutional. 

In a defence filing, lawyers for Trump wrote: “The 45th president respectfully requests the Senate to acquit him on the merits of the allegations raised in the article of impeachment.”

The lawyers for Donald Trump also plans to lean on the US Constitution in multiple ways, including arguing that Mr Trump enjoyed First Amendment protections in everything he said to his supporters.

"The fatal flaw of the House's arguments is that it seeks to mete out governmental punishment - impeachment - based on political speech that falls squarely within broad protections of the First Amendment," the lawyers say.

HeraldScotland:

What happens next?

With representatives now likely to hear the case on the possible impeachment of Donald Trump, each side will now have 16 hours to present their case. This is likely to be done over 8-hour sessions. The process will involved opening arguments, and senators will then be able to question sides by submitting written questions. Debates could also lead to votes on calling witnesses or subpoenaing documents before moving onto closing arguments. 

READ MORE: Stage set for Trump impeachment trial

The defence will start at 17:00 GMT.

It is understood that both sides are keen for a speedy trial and a decision could be made by Monday 15th. 

Is Donald Trump likely to get acquitted?

As mentioned, 45 Republicans signed a motion stating that the impeachment was unconstitutional - indeed, this motion to actually carry out the trial only passed by a slender majority.  Only 6 Republicans sided with the Democrats that the trial should continue. In order to impeach Trump, 17 Republicans would need to side with their opposing party. 

The early vote shows the mood on the floor of the chamber and a larger sway towards the Democrats. Much has been made of the vote of Mitch McConnell, who has been seen by many as an influential Republican who could sway some votes.

Ten Republicans in the House supported impeachment and a handful of senators could be swayed, however, the vote yesterday implied it would be a tall order for an impeachment vote to be successful.

The entire senate will hear the case, if there is a split, Vice President Kamala Harris could have the tie-breaking vote. 

HeraldScotland:

READ MORE: Trump impeachment: Senators to hear opening arguments

Who are the key players?

Aside from the Senate Minority Leader mentioned above in Mitch McConnell, there are also a number of key players in the process. Patrick Leahy of Vermont, the president pro-tempore of the Senate will oversee the impeachment trial. His role will be similar to a judge and could rule on certain questions. 

Trump will have a legal representation made up of attorneys David Schoen, Bruce Castor Jr. and Michael van der Veen.

There are nine impeachment managers, who essentially act as the prosecutors in the case. They are lead manager Jamie Raskin, Diana DeGette, David Cicilline, Joaquin Castro,  Eric Swalwell, Ted Lieu, Joe Neguse, Madeleine Dean and Delegate Stacey Plaskett. All 100 US senators have been sworn in as jurors in the trial.

HeraldScotland:

What happens if Trump is convicted?

While Donald Trump does not need to worry about being removed from office, the Senate could hold a vote if he is convicted to prevent him from ever holding office again. Such a vote would only need a basic majority. 

Will Trump give evidence?

Donald Trump has rejected calls to give evidence at the trial. Prosecutors sent a letter inviting Trump to testify under oath "at a mutually convenient time and place". 

While this has been rejected, prosecutors do have the power to issue a subpoena to force Trump to testify or appear in court. However, this is highly unlikely.

Trump impeachment: When will we get an outcome?

As mentioned, the impeachment trial is being fast-tracked of sorts, and a decision can be expected around Monday 15th.