Czech President Milos Zeman has apologised to Serbia for the 1999 NATO air strikes against the former Yugoslavia.
The strikes, which lasted three months, were aimed at forcing former Belgrade strongman Slobodan Milosevic to abandon a violent crackdown on Albanian separatists in Kosovo.
The US-led campaign effectively ended the Kosovo War, which killed more than 13,000 people.
Milos Zeman, who was Czech Prime Minister from 1998 to 2002 during the bombing campaign, said his country had been "the last" in NATO to agree to the principle of air strikes.
"I would like to take this opportunity to apologise for the bombing of the former Yugoslavia," Zeman said during an official visit to Prague by Serbian President Aleksandar Vucic on Tuesday.
The 76-year-old Czech president, who faces domestic challenges over his ties to Russia, added that his country had "desperately" sought partners in the alliance to oppose the bombing but had failed.
"It was a lack of courage," Zeman said on Tuesday.
"With this request for forgiveness, I am resolving a long-standing trauma because remorse is liberating. I said it and I saved my soul."
President Vucic said Serbs would be "eternally grateful" to Zeman for the apology.
"The words he just said about the strikes have never been uttered before," Vucic added.
The 11-week air campaign was the first military intervention by the Alliance against a sovereign state in NATO's 50-year history.
Serbia does not recognise the 2008 declaration independence of its former province, Kosovo.