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On one level it all seems rather silly - a tit-for-tat squabble worthy of the playground.

Croatia blocks freight traffic from Serbia, Belgrade responds in kind, then Zagreb ups the ante by barring any Serbian-plated vehicle from entering its territory.

Serbia has long since called for teacher - in the form of the European Union - and if there had been a ball someone certainly would have huffed off home with it by now.

Zagreb is trying to use its border controls as leverage to persuade Belgrade to direct the flow of migrants somewhere other than Croatia - understandable after around 50,000 arrivals in a week.

For its part, Serbia has justifiably stated that Croatia's initial, unilateral action violated the EU's Stabilisation and Association Agreement with Belgrade.

The hurt remains

But there are serious notes to this silly symphony. There is still anger and resentment on both sides dating back to 20th Century conflicts. Few can now remember World War Two - but the Croatian War of Independence only ended in 1995.

Last month's 20th anniversary of Operation Storm, which effectively brought the conflict to a close, was marked by huge celebrations in Croatia.

That caused massive offence in Serbia, where people remember how hundreds of thousands of ethnic-Serb civilians had to flee for their lives, while others were murdered.

Belgrade has also played the Nazi card, comparing the recent border-blocking shenanigans with the behaviour of the Croatian fascist Ustase regime during World War Two.

That is a serious escalation of rhetoric, considering the Ustase systematically massacred tens of thousands of Serbs, Roma and Jews.

On the Croatian side, just the name of the county into which most of the refugees are crossing from Serbia says it all: Vukovar.

Serb forces pulverised the main town in 1991, massacred civilians and soldiers and forced ethnic-Croats to leave what they called the Republic of Serbian Krajina.

Other Croatian cities suffered massive damage during the conflict, notably Dubrovnik, whose Unesco-listed old town was bombarded by Serb forces.

Relations had improved over the past 15 years, as Serbia embraced democracy and Croatia moved away from hard-line nationalism and towards membership of the European Union.

Bilateral ministerial visits became almost routine - and Croatia promised to help Serbia in its own application for EU membership.

But the border row indicates how much hurt remains.

There could be more turbulence in relations between the two countries before they find themselves fellow-EU members - especially if Croatia's election of a nationalist president, Kolinda Grabar-Kitarovic, is followed by victory for her HDZ party in forthcoming parliamentary elections.