Kosovo's police have detained several people, including two UN officials in the Serb-dominated north.
Problems between Serbia and Kosovo go back 10 years to Kosovo's independence, back to the war of the late 1990s and arguably back well beyond that too.
On Tuesday, Serbia put its troops on full alert, after armed Kosovan police entered a Serb-dominated area in the north, fired tear gas and - according to some reports - live ammunition, and arrested 23 people.
Kosovo called it an operation against smuggling and corruption, and the arrests included two members of the UN mission in Kosovo.
Serbia's President Aleksandar Vucic said he wants to "preserve peace and stability", but that Serbia "will be fully ready to protect its people at the shortest notice".
So, was it a crackdown on crime or an act of provocation?
Dušan Janjić - A former Serb politician, and now head of the Forum for Ethnic Relations.
Cvete Koneska - Associate Director of the consultancy group Control Risks.
Bodo Weber - Senior Associate of the Democratization Policy Council, and a specialist on Kosovo-Serbia relations.
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