When I read yesterday that Jeremy Corbyn could not see “what there is to commemorate about the first world war”, I thought at once of my great-uncle Jack Arnot. A mining engineer who joined up early in the conflict and was a sergeant in the Northumberland Fusiliers, he was shot at the Somme in 1916, and died after 30 days struggling for life. That seems worth commemorating to me, if not to the Labour leader.

I am sure that Corbyn’s allies will point out that his remarks were made in April 2013, more than two years before he succeeded Ed Miliband, and that they were inspired precisely by a sense of outrage at the “mass slaughter of millions of young men on the western front and all the other places” and that the true target of his invective was the Cameron regime. “The government,” he objected, “is apparently preparing to spend shedloads of money commemorating the first world war” – which he dismissed as “a war of declining empires”.

On Remembrance Sunday, Corbyn will take part in the wreath-laying at the Cenotaph – mindful, no doubt, of the price Michael Foot paid for wearing a short blue-green coat mistaken for a donkey jacket at the 1981 ceremony. Corbyn has already been compelled to promise his nervous parliamentary party that he will not be sporting a white poppy. To become the leader of one of the main parties is to sign up for a crash course in political symbolism. In our overwhelmingly visual culture, with its brocade of viral images, a successful politician has to be a deft semiotician.

That’s style: what about substance? In his book on the first world war, Jeremy Paxman writes aptly that the conflict was “the great punctuation point in modern British history” and “the entire nation has been conducting a form of seance ever since”. It haunts us to this day because, in sharp contrast to the significance of the second world war, its meaning and morality are still fiercely contested.

Those who believe that the 1914-18 conflict, for all its horror, was justified probably have the edge – just – in contemporary scholarship. The prewar belligerence of Germany, the Napoleonic ambition of Kaiser Wilhelm II, and his desire for continental hegemony were such that it is hard to envisage a peaceful resolution to these tensions would have looked like. Britain’s fear of a radically enlarged German navy roaming the North Sea and French ports was real – and justified.

The red poppy is not a badge of militarism, but a token of the bond between the living and the dead

Not that these contentions have settled the debate. In his own remarks, Corbyn echoed the familiar argument of the Marxisant left – according to which the so-called “Great War” was an industrial conflict between rival imperialist-capitalist blocs, ended by the surge of class-based sentiment triggered by the Russian revolution in 1917. What is so striking about the first world war is that it has long stirred a symmetrical hostility in the collective folklore of the right. For many traditionalists the conflict was the moment that a generation of the ruling class was wiped out, and that the Tennysonian era of muscular Christianity, government by classicists and imperial idealism came to an end in the sights of machine guns mowing down 723,000 Britons.

“To many of us,” wrote the Tory maverick Alan Clark in the introduction to The Donkeys (1961), “the first [world war] is as remote as the Crimean, its causes and its personnel obscure and disreputable.” In his magisterial book The Pity of War (1999), the conservative historian Niall Ferguson declared that the British intervention in 1914 was “nothing less than the greatest error of modern history”, unjustified by the scale of the German threat, and that the British government “ultimately decided to turn the continental war into a world war”.

At a more visceral level there is a resilient strain of Toryism that has never recovered from the 1914-18 conflict and, in the years since the Iraq war, has rebelled not only against the liberal interventionism of the Blair era but also against David Cameron’s intermittent desire to commit British forces to battle. Though Ed Miliband will never be forgiven by the PM’s circle for the failure of the 2013 vote on military action in Syria, it should not be forgotten that 30 Tory MPs also mutinied – including David Davis, Crispin Blunt and Tracey Crouch, who is now the sports minister.

These Conservatives, no less than Miliband, sabotaged Cameron and shattered Barack Obama’s fragile commitment to intervene. If, as seems probable, the prime minister asks the Commons again for a green light to take action in Syria, he will face the same struggle. At the Tory conference one senior minister admitted to me that “so far, the numbers are not there”.

There are still Tories who, to borrow Douglas Hurd’s phrase, want Britain to punch above its weight on the geopolitical stage. A handful, some senior in the government, could even be plausibly described as neocons, hawks or Wilsonian interventionists (Woodrow, not Harold). Although they are dwindling in number, it is no accident that Michael Gove described the 1914-18 conflict as “a just war”, deploring the Blackadder orthodoxy that prevails in popular sentiment, if not in scholarly debate. Gove’s historical opinions mesh with his intellectual position on contemporary foreign policy.

No, Corbyn’s error was not his verdict on the war’s morality. To echo Wilfred Owen and Siegfried Sassoon and their unforgettable descriptions of mechanised death and “doomed youth” is scarcely unusual or improper. It is not intrinsically shocking to the British to hear a politician say that the conflict was a senseless slaughter.

What was foolish was his confusion of the war and its commemoration. To spend “shedloads of money” on acts of recollection and tribute was not a waste of public funds, but a means of keeping alive memories ever more at risk in our amnesiac culture. How we remember is no less important than how we dream.

The red poppy is not a badge of militarism, but a token of the bond between the living and the dead. What jars is Corbyn’s apparent inability to see that remembrance is not a celebration of warfare, but a collective expression of gratitude to those who served and to those who fell. It is no exaggeration to say that a politician who cannot see the difference is unfit to be prime minister.