Paddy Ashdown, the former leader of Britain’s Liberal Democrats and former United Nations high representative for Bosnia, died Saturday evening after struggling with a short illness, according to a tweet from the Liberal Democrats’ verified Twitter account. He was 77.

“This is a hugely sad day for the Liberal Democrats and for the very many people across political and public life who had immense affection and respect for Paddy,” said Vince Cable, current leader of the party. “He took up unpopular causes where he was respected for his convictions, in particular promoting the rights of the citizens of Hong Kong, and — later — military intervention in Kosovo.”

British Prime Minister Theresa May said he served his nation with great distinction.

“He dedicated his life to public service and he will be sorely missed,” she said. “My thoughts and condolences are with his family and friends at this difficult time.”

Ashdown served in the Royal Marines from 1959 until 1972. After he left the military he joined the UK Foreign Office for four years.

He was first elected to office in 1983. Five years later he was elected leader of the Liberal Democrats, a position he held until 1999. He retired in 2001.

He was the UN high representative for Bosnia and Herzigovina from May 2002 until January 2006, responsible for implementing the 1995 Dayton Peace Accord that ended four years of armed conflict in the country.

CNN’s Christiane Amanpour, who interviewed Ashdown many times, on Saturday called him a “gentleman, soldier, politician.”

“I first traveled to Bosnia with him to investigate war crimes. His unique brand of diplomacy will be missed,” Amanpour wrote on Twitter.

In 2017, Amanpour caught up with Ashdown shortly after former Bosnian Serb army leader Ratko Mladic was convicted of genocide for atrocities committed during the Bosnian war in the 1990s.

Ashdown investigated genocide during the war and told Amanpour, “I didn’t think they’d get him (Mladic) when I was in Bosnia. I worked very hard to make sure that we set the context in which he could be captured. So I was delighted that he was and delighted that this long process — careful, steady, meticulous process — be brought to an end and this man is where he should be, in jail for the rest of his life.”