Starving, dehydrated and in many cases, dying, hundreds of African slaves are crammed on to the deck of a British ship. Taken in the early days of photography, this picture gives a disturbingly contemporary feel to one of history's bleakest chapters.

No wonder the National Archives chose it to illustrate the 'profoundly oppressive' nature of the British Empire in an exhibition.

Thousands of visitors, many of them schoolchildren, would pause to reflect on the photograph captioned: 'East African slaves taken aboard HMS Daphne from a dhow, 1 November 1868.'

The subtext was clear: look at the cruelty of these brutal British imperialists. Except the curators of the exhibition had made one very serious mistake indeed. For the poor souls in this picture were no longer slaves.

They had just been rescued by HMS Daphne from an Arab slave ship, liberated from a crew of monsters, one of whom had beaten a baby to death a few hours earlier.

Thousands of visitors would pause to reflect on the photograph (pictured) captioned: 'East African slaves taken aboard HMS Daphne from a dhow, 1 November 1868'

That British sailor in the bottom left of the photograph was no villain. He was one of the good guys, part of a high-risk Royal Navy operation to curtail a roaring slave trade operating out of East Africa. It was a despicable business involving a network of Arab potentates plus Portugese, French and American traders.

The British, who had first voted to abolish the slave trade more than 60 years earlier, were trying to stop it — not that it suits the Empire-bashing narrative to say so.

The true story behind this extraordinary photo is told in a long-forgotten Victorian memoir called Dhow Chasing in Zanzibar Waters — Five Years' Experience In The Suppression Of The Slave Trade. It is written by the man who took this picture, Captain George L. Sulivan, HMS Daphne's skipper. He was no cold-hearted imperialist.

True, some of his 19th-century language would have him denounced as a racist by today's campus commissars.

But there is no mistaking the motives of a man with a burning passion to stop this 'nefarious' trade in the 'vile cesspools' of the Indian Ocean. Thanks to Capt Sulivan and his men — several of whom died in the process— thousands of enslaved Africans were freed.

The days before his photo was taken involved a series of horrific episodes, including an attack by Somali tribesmen while Capt Sulivan and his men were rescuing several emaciated slave children from a wrecked Arab dhow.

Then, on November 1, the crew of HMS Daphne, a 187ft sloop with a crew of 150, intercepted another dhow and discovered 48 men, 53 women and 55 children imprisoned on board.

'The deplorable condition of some of these poor wretches, crammed into a small dhow, surpasses all description; on the bottom of the dhow was a pile of stones as ballast, and on these stones, without even a mat were 23 women huddled together — one or two with infants; these women were literally doubled up, there being no room to sit erect.

No wonder the National Archives (pictured) chose it to illustrate the 'profoundly oppressive' nature of the British Empire in an exhibition, writes Robert Hardman

'Some of the slaves were in the last stages of starvation and dysentery.

'A woman came up having an infant about a month or six weeks old in her arms with one side of its forehead crushed in.

'On asking how it was done, she told us that just before our boat came alongside the dhow, the child began to cry and one of the Arabs, fearing the English would hear it, took up a stone and struck it . . .' HMS Daphne captured two more dhows that same day.

Conditions on the British ship were certainly basic and would get worse following an outbreak of smallpox which struck both the liberators and liberated.

But everyone was fed, washed and attended by the ship's doctor. A few weeks later, more than 300 were put ashore in the Seychelles to begin a new life of liberty while HMS Daphne returned to active duties.

We must never forget the wrongs committed by the British Empire. But every single person in this photograph certainly had good reason to be thankful to it for evermore.