Less than two years ago, Johnny Depp lost a UK libel case against the Sun after his ex-wife Amber Heard gave evidence to back claims in the newspaper that he was a wife-beater.

The judge, Mr Justice Nicol, said the Sun had proved its article to be “substantially true” and found that 12 of 14 alleged incidents of domestic violence against Heard had occurred.

At the time, Heard’s lawyer in the US, Elaine Charlson Bredehoft, said the ruling was no surprise. “Very soon, we will be presenting even more voluminous evidence in the US,” she said.

Bredehoft may well be surprised now. On Wednesday, Heard’s evidence was roundly rejected by a jury in Virginia despite it being assumed by many, including legal experts, that Depp had a weaker chance of victory.

The perceived wisdom is that it is much easier to win a defamation case in the UK than in the US, where the enshrinement of free speech in the first amendment of the constitution is sacrosanct. So what happened in this case to reverse the expected result?

Mark Stephens, an international media lawyer, said the fact that the US case was heard before a jury while the UK trial was heard before a judge was significant.

“Because the US trial was before a jury, it allowed Depp’s lawyers to focus on Heard,” a well-worn tactic of defendants in domestic abuse cases but one that was dismissed by the judge in the UK, Stephens said.

“They deny that they [their client] did anything, they deny they’re the real perpetrator, and they attack the credibility of the individual calling out the abuse, and then reverse the roles of the victim and the offender.”

Heard’s team also made tactical mistakes and were outdone by a more experienced set of lawyers, Stephens added. “Heard’s team were not predominantly trained libel lawyers and they were outgunned at every corner. They were up against a very strong team for Depp,” he said.

There were also claims that the use of “Darvo” tactics – when the alleged offender denies the behaviour, attacks the accuser, and reverses the roles of victim and offender – was not confined to the courts.

According to the US academic who coined the term Darvo and has studied the tactics used by alleged sexual predators, social media was used to undermine Heard’s case and bolster Depp’s.

Jennifer Freyd, a professor emeritus of psychology at the University of Oregon, said the traducing of Heard’s reputation online was “overwhelming”.

“Darvo refers to a reaction [that alleged] perpetrators of wrongdoing, particularly sexual offenders, may display in response to being held accountable for their behaviour,” she said.

“This occurs, for instance, when an actually guilty perpetrator assumes the role of ‘falsely accused’ and attacks the accuser’s credibility and blames the accuser of being the perpetrator of a false accusation.

“What we have witnessed in the US over this case has been an overwhelming case for Depp on social media. It is like an anti-Heard campaign and there has been a lot of Darvo.”

On TikTok, the hashtag #justiceforjohnnydepp received 19bn views. Jurors were instructed not to read about the case online but they were not sequestered and they were allowed to keep their phones.

Persephone Bridgman Baker, a partner at libel specialists Carter-Ruck, said that ultimately the jury believed Depp. “There seems no more obvious explanation than that the jury simply believed Depp’s evidence in the US proceedings, or, if you accept that a Darvo strategy was employed, that the jury accepted it.

“There was more evidence in the US proceedings about Heard’s credibility, on which the judge in the UK placed little importance: that is likely to have been a deliberate strategic decision by Depp’s team. While the judge in the UK proceedings decided Heard was a credible witness, that additional evidence may have swung a jury,” she said.

Will the US result have consequences for other women who wish to make claims of harassment or abuse against high-profile individuals, anywhere in the world? Freyd said it will. “Others who wish to make claims will see what has happened in this case, and see what has happened to Heard, and think twice. Many will be afraid to talk,” she said.