A visitor to London would not expect to see a car sticker showing the British empire from India to Australia. A tourist in Paris would be equally surprised to see a map of France including Algeria and Tahiti. Yet a decal of Greater Hungary is a surprisingly common sight on vehicles in Budapest. Before Hungary lost two thirds of its territories at the Treaty of Trianon in 1920, its borders, as part of the Austro-Hungarian empire, reached deep into present-day Romania, Slovakia, Serbia, Ukraine and Croatia. Almost a century later, the scars of Trianon are still raw, which is why a diplomatic war has erupted between Hungary and Romania. Earlier this month, Titus Corlatean, the Romanian foreign minister, threatened to expel Oszkár Füzes, the Hungarian ambassador to Romania. Mr Füzes had voiced support for the Székler people, a group of ethnic Hungarians who live in Transylvania, after two Romanian counties banned the display of the Székler flag (pictured above with men in hussar uniform) on public buildings. Zsolt Nemeth, Hungary’s state secretary for foreign affairs, described the ban as an act of “symbolic aggression” and called for local councils in Hungary to show solidarity by flying the Székler flag from town halls. The Hungarian government then raised the Székler flag above Parliament, further enraging Bucharest.

The Széklers have long demanded some kind of autonomy. Hungary says that the Romanian authorities should consider their request and work towards a solution. But the Romanian government flatly rejects the idea. Bucharest fears that autonomy for ethnic Hungarians would soon lead to a declaration of independence and the break-up of the Romanian state.

Romania is home to at least 1.5m ethnic Hungarians. Many in Romania, and other countries home to ethnic Hungarians, are already unhappy that the Hungarian government has granted citizenship to almost 400,000 Hungarians ‘beyond the borders’ as they are known in Hungary. The new Hungarians citizens can vote in the next general election, scheduled for spring 2014, thus intensifying accusations of ‘dual loyalties’.

Hungary rejects such claims, arguing that as the Hungarians beyond the borders are part of the Hungarian nation, they should have a say in its future. Hungary, they say, has no territorial claims on its neighbours. János Martonyi, Hungary’s foreign minister, who was himself born in Transylvania, has called for calm between the two countries, hoping that the “din of battle would subside”.

Yet it seems some nationalistic Hungarians in positions of power still regard the lands lost at Trianon as part of their fiefdom. Last May tension soared between Hungary and Romania after Laszlo Kövér, the firebrand speaker of parliament, spearheaded an attempt to rebury the remains of József Nyirő, a far-right writer, in his hometown in Romania. Nyirő was a former Catholic priest who served as an MP during the Arrow Cross Hungarian Nazi regime at the end of the Second World War and died in exile in Spain in 1953.

Nyirő’s remains were brought back to Budapest. His books are now included in the school curriculum. Hungarian officials said Nyirő’s writings should be separated from his politics, rather as Ezra Pound’s sympathy for fascism does not detract from the quality of his poetry.

The Romanian government did not agree. It denounced him as an anti-Semite and promptly banned the reburial ceremony.It is now becoming clear that the dispute between Hungary and Romania over the Treaty of Trianon was only deep-frozen during the Communist era. More than twenty years after the change of system, democracy, it seems, has not brought more understanding between Budapest and Bucharest, but only more opportunities for populist gestures and unedifying squabbles.