A group of Ethiopian treasures, now on special display at the Victoria and Albert Museum, have rekindled an old debate about whether such artefacts should be returned to their country of origin. Ethiopia would like them back. The V&A’s director, Tristram Hunt, has suggested a long-term loan. The Ethiopian government has welcomed Hunt’s offer. And there, for now, the matter rests.

The treasures, which include an 18th-century gold crown and a royal wedding dress, are part of a fascinating and beautiful collection with a tragic and little-known history. They were seized by the British in 1868 and have been in the V&A ever since. But the circumstances of their seizure, and the consequences, deserve to be much better known.

Even most of those who know something of British imperial history are unlikely to know much about the 1868 invasion of Abyssinia which lies at the root of the current debate about the future of these treasures. That’s because the Abyssinia campaign barely features in the dominant historical narratives about British imperial power. The discredited heroic narrative of imperial Britain had prominent places for the conquest of north America, India and even South Africa. The critical big post-colonial narrative that replaced it concentrated on India, the Middle East, the Atlantic slave trade and even Ireland. But the Abyssinia campaign has remained on the margins of both.

The 1868 war gets a passing ironic mention as one of the “justifiable wars” of the 19th century in 1066 and All That. The tragic and poignant fate of the Abyssinian Prince Alemayehu, who was brought to Britain with the treasure in 1868, is possibly marginally better known than the events that gave rise to it. Alemayehu met Queen Victoria and was sent to Rugby school. He died in Headingley, Leeds, at the age of 18 while staying under the care of his former tutor Cyril Ransome, father of the children’s author (and Guardian journalist) Arthur Ransome, and is buried in Windsor Castle. Alemayehu’s exile has recently been the subject of a film and a radio programme. But the wider truth is that this is a forgotten story – except in Ethiopia.

Abyssinia, now Ethiopia, was on the wrong side of Africa to be caught up in the Atlantic slave trade, though always vulnerable to the Arab slave trade. It was too mountainous and isolated to be routinely involved in the struggles between Britain, France and Turkey to control the Red Sea and the route to India. And when the European colonial “rush for Africa” began in earnest in the mid-19th century, it offered too few easy pickings in terms of minerals, cash crops and settlement.

Nineteenth-century Abyssinia, with its ancient culture and its dominant Coptic Christian religion, was both a place apart for the European powers and a place with real, if uneasy, connections to the neighbouring Ottoman Turkish world that dominated north-east Africa and the Middle East. When, in 1855, a remarkable local ruler called Kassa unified Abyssinia’s many provinces and crowned himself the Emperor Tewodros, the outside world began to be aware of one of the great African leaders of the 19th century.

Tewodros was a man of enormous energy and ambition. His aim, never fulfilled, was to create a modern nation state. Though he always struggled to maintain control, he abolished feudal dues, paid salaries to judges and local governors, outlawed bribery, confiscated firearms, forbade the slave trade and abolished polygamy. His growing power attracted foreign governments. A British diplomat visited him in 1855. And in 1862, after receiving a present of two pistols from Britain, Tewodros wrote to Queen Victoria with an audacious proposal. As the great Christian emperor of the south, he proposed an alliance with the great Christian empress of the north. Together, their armies would unite – and drive the Muslim Ottomans from the holy places in Jerusalem.

It was a fateful letter. Lord Palmerston’s government in London had no intention of getting involved. The letter was ignored. Tewodros responded by taking hostage the handful of Europeans in the country, including a British emissary. For years, efforts were made to negotiate the hostages’ release. But Britain’s refusal to send aid workers or to join in the Jerusalem adventure made that impossible. The situation festered and Tewodros’s power began to decline. In 1868, in an operation costing £9m – the equivalent of more than £1bn today – Britain sent an army of 13,000 troops. In April of that year, the British overwhelmed Tewodros’s army at the fortress of Maqdala. In a scene worthy of the climax of a Verdi opera, Tewodros committed suicide with one of the pistols from Queen Victoria.

Tewodros was unquestionably a tragic victim of British imperial might

With Tewodros dead, the British withdrew. The troops went back to India. The freed hostages went home. The railways that had been built to carry military equipment up from the Red Sea coast was pulled up. The Maqdala treasures, along with Tewodros’s son, were taken to Britain. The prime minister, Benjamin Disraeli, told MPs that “We are about to vacate the country in a manner which will prove to the world the purity of our purpose.”

The Abyssinia war is an unusual episode in the British imperial story. It was not a war of plunder, of imperial rivalry or annexation. Politicians at the time thought it was a war about prestige. The British did it because they could. They had the wealth and the weapons; the Ethiopians had neither. Though Tewodros never ruled a stable Abyssinia, his overthrow weakened the fragile state at great human cost. Tewodros was unquestionably a tragic victim of British imperial might.

Abyssinia would be invaded twice more, defeating the Italians in their historic victory at Adowa in 1896, and then being brutally attacked by Mussolini in 1935. Today, Disraeli’s claims of moral purity are not words that anyone would use. And nor should they rule out the return of the Maqdala treasures.

• Martin Kettle is a Guardian columnist