The President of the European Commission, 62, warned the United States government to stop stirring up trouble, after Donald Trump praised Brexit.
He spoke with Vice President Mike Pence and said: “Do not say that, do not invite others to leave, because if the European Union collapses, you will have a new war in the western Balkans.”
“If we leave them alone — Bosnia Herzegovina, Republika Srpska, Macedonia, Albania, all those countries — we will have war again.”
The president of the European Commission launched the stinging attack on the US president by criticising him for encouraging other countries to copy Brexit.
He then branded Trump’s enthusiasm for seeing Britain leave the EU “annoying”, in the interview with the Financial Times.
He added it showed the real estate tycoon was unaware of the complicated history of Europe.
Mr Juncker also said the US president is scaring Europeans with his rhetoric.
He added: ”For the first time in postwar history we have an American president giving the impression he is not interested in European affairs.”
At the end of the Cold War in 1991, the break-up of former Yugoslavia led to a brutal ethnic war between Serbs, Croats and Bosnian Muslims.
It only ended after a Nato-led military intervention.
Juncker also said that countries in the region should be entitled to the prospect of having EU membership.
He warned that if they are left alone and isolated, it could lead to a raging conflict again.
The commission president will visit Washington next month. But he does not know if he will meet with Trump.
He believes that the American president has other priorities aside from meeting him.
“They’re trying to fix it, but he has other priorities. By the way, he does not understand anything about Europe,” he said.
“He had Tusk [Donald Tusk, president of the European Council] on the phone and he thought it was me.”