Tensions between Croatia and Serbia escalated rapidly over the weekend, after the authorities in Zagreb did not allow Serbian President Aleksandar Vucic to visit the former Jasenovac concentration camp.

Jasenovac was a concentration and extermination camp established in Croatia by the authorities of the Independent State of Croatia (NDH) in occupied Yugoslavia during World War II. It was one of the ten largest concentration camps in Europe and was established and operated by the governing Ustase regime, which was collaborating with the Nazis. It was intended for Serbs, Jews and other ethnic groups.

According to Jutarnji List daily, Vucic announced his intention to visit the camp privately but the Croatian authorities decided this was unacceptable.

Vucic informed the leader of the Croatian Independent Democratic Serb Party (SDSS), Milorad Pupovac, of his plans. Pupovac, in turn, decided to inform Croatian Prime Minister Andrej Plenkovic of Vucic’s plan, which was seen as a provocation by Zagreb. Officials claimed the visit was aimed at causing chaos in Croatia. 

Foreign Minister Gordan Grlic Radman said as quoted by N1 that the fact that the Croatian government had not been officially notified of the visit was unacceptable. 

“The Ministry of Foreign Affairs would like to stress that in the planning of any visit by foreign officials the time, nature and programme of the visit should be subject to official communication and agreement by both sides,” Radman said as quoted by Associated Press.

“This was not a trip to the seaside. The president of a country is a protected individual,” he added.

Radman also said that the decision to visit the country informally without coordinating with the local authorities was very unusual and provocative.

“We consider this a provocation. The victims here are not the goal, but the means. The aim is to present President Vucic's efforts to the Serbian public, to get additional political points as the new Serbian government is formed,” Radman said.

Jutarnji List quoted sources from the Croatian government as commenting that Vucic did not respect any diplomatic protocol as he had not informed the Croatian authorities, nor had he coordinated his plans with any embassy.

The decision to block the trip angered Belgrade and Serbian Prime Minister Ana Brnabic called it the biggest scandal in the modern history of Serbia and Croatia.

“It is not clear to me whether he was banned from visiting Croatia or Jasenovac and if he could go anywhere else except to Jasenovac, which, I think, is an incredible precedent,” Brnabic said as quoted by N1.

She added that this was a “brutal violation of the freedom of movement”.

She said that now Vucic will file a formal request for a visit to check whether Croatia will allow him to visit the concentration camp.

Serbia’s Interior Minister Aleksandar Vulin said on July 17 as quoted by N1 that from now on public officials from Croatia will be obliged to specifically announce and provide reasons for their visits or passing through Serbia and will be put under a special regime of control. Radman described Vulin's statement as scandalous.

At the same time, Serbia’s Foreign Affairs Minister Nikola Selakovic said on July 17 that the country will respond with measures against Croatia but did not specify its planned steps.

Serbia’s Mining and Energy Minister Zorana Mihajlovic cancelled her planned visit to Croatia following Zagreb’s decision not to let Vucic enter the country unofficially.

Spats have previously erupted between the two neighbouring countries over Jasenovac and Croatia’s WW2 history. 

In 2018, Belgrade and Zagreb threw barbs at each other over a Serbian-Jewish exhibition about the concentration camp at the United Nations headquarters in New York, called "Jasenovac - the Right Not to Forget".

Two years earlier, a Zagreb court’s decision to annul the 1946 conviction of Catholic cardinal Alojzije Stepinac, known as a supporter of the Ustashe pro-Nazi regime, for treason on the grounds that he did not receive a fair trial was condemned by Serbian officials.