When Will Smith made a guest appearance last week on Comedy Central’s The Daily Show, there was no avoiding The Slap – his eruption of anger at comedian Chris Rock during the Oscars. But first, host Trevor Noah wanted to talk about Smith’s latest film: “You play one of the most famous unknown people … from America’s history.”

That person is today known as “whipped Peter” or by his given name as an enslaved man, Gordon. A photograph showing his scarred back after his escape from a Louisiana plantation in the 1860s became one of the first viral images, displaying the brutality of slavery to a world that could no longer look away. Peter’s story is the inspiration for Smith’s comeback movie, Emancipation, which begins streaming on Apple TV+ on Friday.

The backdrop is the American civil war. On 1 January 1863, President Abraham Lincoln issued the Emancipation Proclamation declaring “that all persons held as slaves” within states that had seceded from the union “are, and henceforward shall be free”, though it left slavery untouched in loyal border states. The proclamation also announced the acceptance of Black men into the Union army.

Mary Elliott, curator of American slavery at the Smithsonian National Museum of African American History and Culture in Washington, says in an online call: “This war was about slavery, about the ability to maintain slavery, and Lincoln was about keeping the union together.

“Ultimately, he passes the Emancipation Proclamation which freed enslaved Black people only in the rebelling states. That’s a very important distinction. A state like Maryland did not leave the union and still maintained slavery. That meant slavery hadn’t ended throughout the whole nation.”

Peter was in one of the southern states that had rebelled. He was among 40 enslaved people on John and Bridget Lyons’ expansive cotton and onion plantation in St Landry parish, Louisiana. They were subjected to unspeakable cruelty.

In March 1863 Peter staged an extraordinary escape. For 10 days he was on the run, hunters and dogs at his back, disguising his scent with onions and navigating the treacherous bayou. Forty miles later, he finally reached the Union army stationed in Baton Rouge – he was finally a free man.

But before enlisting in a Black regiment, he was examined by military doctors. A witness told the New York Daily Tribune newspaper: “He pulled down the pile of dirty rags that half concealed his back. It sent a thrill of horror to every white person present, but the few blacks who were waiting paid but little attention to the sad spectacle, such terrible scenes being painfully familiar to them all.”

The shocking sight was captured by New Orleans-based photographers William McPherson and J Oliver. In the portrait, which became known as “The Scourged Back”, Peter sits with hand on hip, back to the camera, glancing over his shoulder. His skin is riven with a crisscrossed map of keloid scars from a whipping by a plantation overseer, an act of abuse that Peter said left him in bed for two months.

According to the New York Daily Tribune, he told the Union troops on 2 April 1863: “Overseer Artayou Carrier whipped me – I don’t remember the whipping. I was two months in bed, sore from the whipping and salt brine, which Overseer put on my back. By and by my senses began to come – they said I was sort of crazy, and tried to shoot everybody.”

The photo was published in Harper’s Magazine and created a sensation. It was circulated fervently around the country by abolitionists and published worldwide. Unlike written accounts of slavery, all too often dismissed as unverifiable, the photograph provided incontrovertible and visceral evidence.

Theodore Tilton, a newspaper editor, wrote that it “should be multiplied by the hundred thousand and scattered over the states … If seeing is believing – and it is in the immense majority of cases – seeing this card would be equivalent to believing things of the slave states which Northern men and women would move heaven and earth to abolish!”

The image can be seen a forerunner of mobile phone videos that now capture police brutality against people of colour, spurring the Black Lives Matter movement. Elliott adds: “People try to tell themselves things to make themselves like, ‘Well, slavery wasn’t so bad. The genteel enslaver and the enslaved who loved and cared for their enslaver.’ But let’s just be real. Legally there were opportunities for people to maim a person, even kill a person who was enslaved, who they saw as doing something wrong.

“There were laws in the colonial period where you could split someone’s nose, cut off a limb for different degrees of what the enslavers saw as an egregious act. Legally you could murder someone. There were whipping posts – you could whip someone in public. You can read about it but to see it, that’s a whole different thing. That’s why you can’t turn away. This is real, this man’s back, those lines, those raised lacerations tell a visual story that you can’t turn away from.”

Peter signed up to serve as a soldier in the Louisiana Native Guard, a regiment of the Union army made up entirely of free Black recruits, in an effort to liberate others. He was reported to have fought bravely in the Union assault on Port Hudson in July 1863.

Elliott continues: “That was important for him to be able to do that. His story is powerful because you can’t talk about slavery without talking about freedom. There’s always this desire for freedom. When people say, ‘Oh, another slave film’, ‘Oh, you’re going to talk about slavery’, well, guess what? I’m also going to talk about freedom.”

The trail then goes cold: nothing further is known about Peter’s life. But the photo of him that hangs in the Smithsonian’s National Portrait Gallery continues to agitate, haunt, move, reprove and inspire.

Elliott, 56, who has interviewed Smith about the new film, reflects: “When I see that photograph it makes me think about how it’s important that we’re in touch with our humanity by recognising the inhumanity and we recognise that, with Peter’s story, the strength of a man is endurance, what he experienced, but also the strength of that man is to see the fight through all the way through.

“So now you’ve entered the Union army camp and you’re going to join the fight. To me Black people who did that didn’t just free themselves from slavery. They freed the nation from the bondage of slavery and that cannot be lost on anyone.”

• Emancipation is now out in selected cinemas and will be available on Apple TV+ on 9 December