John Westbrooke reports on this new exhibition at Tate Britain

The Collapse of the Earl of Chatham in the House of Lords, 7 July 1778 comes in the middle of the new Tate Britain exhibition on British history painting.

The earl was a former prime minister and had just been speaking against the calls for American independence when he suddenly clutched his heart and fell. Already 69 years old and frail – his speech had begun “I rejoice that the grave has not closed upon me” – he was taken to his home, where he died a month later.

This was an ideal subject for a classic history painting, a significant point in the life of a great statesman and a great cause. The painter who took it up a year later was John Singleton Copley, American-born himself but recently arrived in England and an admirer of Chatham. He depicted the event on a grand scale, 10ft by 7ft 6in. There are 55 people in it, all individual portraits.

Like any historical TV series, it took some liberties with accuracy in the interests of drama. The peers are all in robes, which they hadn’t been, producing a great wave of red that parts to reveal Chatham, mostly in white and with the light full on him, slumped backwards with his arms out, like a painting of Christ taken down from the cross.

The work made Copley’s name in England. He put it on display privately, charging a shilling for admission (like a raree-show, harrumphed the president of the Royal Academy, clearly unused to the American way of business). He may have hoped that the little individual portraits in it would lead to commissions for full-size ones from the subjects. Engravings from the painting sold well on both sides of the Atlantic.

History painting was seen as the highest form of art and yet, as the show indicates, it can be hard to define. Copley’s Chatham painting wasn’t history at all when he painted it, it was virtually current affairs.

Subjects from ancient history provided a precedent, represented here by tableaus of the King of Sparta sparing the life of a rebellious son-in-law and Agrippina bringing the ashes of her husband Germanicus, son of the Emperor Tiberius, back to Rome.

Those events were at least thought to have happened. But another big oil, Edward Poynter’s A Visit to Aesculapius, seems to have had more than history on its mind: it shows Venus and the Three Graces coming to see the god of medicine about a thorn in the goddess’s foot. This rather minor medical complaint is enlivened by the fact that all four women are attractively naked. Aesculapius, to his credit, keeps his eyes fixed on Venus’s foot; viewers in 1880 may not have done so.

Then there’s King Lear Weeping over the Dead Body of Cordelia (James Barry, 1786), which is purely fictional, taken from Shakespeare, though it does have a little Stonehenge in the background. And as for John Everett Millais’ Speak! Speak! – a young man visited in his bedchamber by a spectral bride – that was simply made up by the artist and looks more Gothic than historic.

Probably the best-known work on display, Millais’ Boyhood of Raleigh, inventively dodges the issue of authenticity altogether by showing the Elizabethan explorer as a child, enthralled by tales of seafaring told to him and his brother by a Genoese sailor with an earring, who points out toward the Spanish main. Victorians loved this sort of thing, perhaps forgetting that it led Raleigh to the executioner’s block; later viewers came to suspect the whole history genre of being imperialist propaganda, whether it depicted Greeks, Romans or Britons.

As well as presenting these big old works, however, the Tate wants to convince visitors that history painting is still alive and well in Britain. The examples it gives don’t entirely bear this out.

Some are striking. In 1932 Walter Sickert painted Miss Earhart’s Arrival not as the triumphant landing of a woman who’d just flown across the Atlantic single-handed, but as a crowd of spectators huddling in the rain: a comment on celebrity rather than aviation. Richard Hamilton’s compelling diptych The Citizen (1981) shows a prisoner, as grim and serious as any classical hero, staging a dirty protest in his cell during the Troubles in Northern Ireland. John Minton’s Death of Nelson is a modernist retelling of an old subject, its dying hero in an even brighter spotlight than the Earl of Chatham was given.

But many of the 21st-century works included show history painting not flourishing but descending into academic agitprop. The first and last paintings visitors see are by Dexter Dalwood, who’s evidently being presented as central to the genre; but both are non-representational and tedious. His vision of Noah’s flood, The Deluge, is most notable for quoting – or just nicking – a windblown tree from Turner’s treatment of the same subject, hung next to it. The comparison does the modern artist no favours. Significantly, the printed catalogue’s cover shows not his work, which might deter potential readers, but a proper 18th-century painting, another of Copley’s.

Entrance to this unpersuasive argument for the future of history painting costs £10.90. The good news is that, as with most exhibitions these days, the cost of insuring and transporting art from elsewhere is prohibitive, so the majority of the show is made up of works in the Tate’s own collection, As a result, from September you’ll be able to see many of them for free again.

Fighting History, Tate Britain, June 9 – September 13 2015.