Kosovo police have been arresting local Serbs who, they say, have attacked NATO peacekeepers and journalists. The police also claim to have found a big weapons cache in the northern bit of Kosovo, where most people are ethnic Serbs. Ethnic Albanians are a large majority elsewhere. American and EU diplomats are crisis-fighting, afraid that another violent incident could blow up into a bigger conflict.

Last November Kosovo’s Serbs resigned from the country’s institutions, including the police. Elections in the Serb-populated north to replace the mayors who had quit were boycotted by Serbs, so ethnic Albanians were elected in their place, often by only a handful of voters. When the new mayors tried to move into their offices in May, local Serbs protested and a mob attacked NATO peacekeepers trying to keep demonstrators back. The Serbian government in Belgrade had hitherto kept a tight grip on Serb paramilitary-cum-criminal groups in northern Kosovo, but has recently seemed less active in doing so.

Mr Escobar and Miroslav Lajcak, the EU’s negotiator, had achieved a breakthrough in the spring by securing an agreement to implement a plan whereby Serbia would treat Kosovo as a state in all but name. In return, Kosovo’s tiny Serb minority would be given a form of autonomy, which they had been promised in an earlier agreement, in 2013.

But Serbia broke the new agreement by trying to block Kosovo’s accession to the Council of Europe. America and the EU then asked Albin Kurti, Kosovo’s prime minister, not to physically install the ethnic-Albanian northern mayors in their town halls. He did it anyway. Backed up by ethnic-Albanian policemen, he is changing the status quo of the past 20 years, alarming Kosovo’s Serbs. A former political prisoner in Serbia, Mr Kurti has found that playing hardball pays dividends.