It is an epic moment in American legal history that played out live on US television – and now new light is shed on the withering exchanges between veteran Boston lawyer Joseph N Welch and redbaiting populist bully Senator Joe McCarthy which effectively put paid to McCarthy’s career.

The famous episode of the McCarthy hearings when Mr Welch, representing the US army, accused McCarthy of being “cruel” and “reckless” is prominently featured in a new PBS documentary on McCarthyism.

A trailer for the documentary explains the background to the moment when McCarthy sought to smear a young lawyer, Fred Fisher, who worked for Hale and Dorr, Welch’s law firm – contrary to an agreement that Welch had with McCarthy’s chief counsel, the appalling Roy Cohn, that he would not mention Cohn’s draft-dodging if Fischer’s previous communist associations were left out.

Cohn, who went on to represent and mentor Donald Trump coaching him in his ‘lie,lie,lie, deny, deny, deny’ approach to public life, squirms as Welch skewers McCarthy with his famous “Have you no sense of decency, sir?” line.

The trailer suggests Welch’s great moment might not have been as entirely spontaneous as popularly believed but it marked the beginning of the end for McCarthy.

Joseph Welch died in 1960, aged 69. After his retirement from Hale and Dorr he played a judge in Otto Preminger’s film Anatomy of a Murder quipping that he did so because it “looked like that was the only way I’d ever get to be a judge”. He took the role on the condition that his wife Agnes got to play the part of a juror.

Fred Fisher became a partner at Hale and Dorr and died in 1989 while lecturing on McCarthyism in Tel Aviv at the age of 68.

Joe McCarthy died in disgrace aged 48 in 1957, of alcoholism. He ruined thousands of lives not only with his red scare campaign but also his ‘lavender scare’ campaign against homosexuals in public life.

Roy Cohn died of AIDS in 1986, aged 59. That same year a five-judge panel disbarred Cohn for misappropriating clients’ funds and lying on a bar application – the least of his many sins.