The International Criminal Court prosecutor said on Thursday it was dropping all war crimes charges against Maxime Mokom, a former militia leader in the troubled Central African Republic (CAR).
Prosecutor Karim Khan said his office had concluded there were "no longer any reasonable prospects of conviction at trial even if the charges were confirmed".
Mokom had faced charges of war crimes and crimes against humanity over alleged atrocities committed against Muslim civilians in the CAR, a former French colony, by his self-proclaimed self-defence militias in 2013 and 2014.
One of the poorest countries in the world, the CAR was plunged into bloody sectarian conflict after Seleka rebels, a coalition of armed groups mainly composed of Muslims, ousted president Francois Bozize in early 2013.
Mokom's militias, which called themselves "anti-Balaka" -- meaning "anti-machete" -- formed in reaction to the takeover of the capital, Bangui, by the Seleka.
He had denied involvement in the bloodshed, telling ICC judges in August that he was "dedicated to the search for peace".
Mokom said he returned to the CAR in February 2014 during the height of the violence after fleeing to the neighbouring Democratic Republic of Congo.
The charges Mokom had faced included directing attacks against the civilian population, murder, rape and pillage.
Prosecutor Khan said the decision to drop those charges was taken after considering all the evidence and "changed circumstances regarding the availability of witnesses".
"I am very conscious that this news may be unwelcome to many survivors and their families," he said.
"I hope many will understand my legal and ethical responsibilities to be guided by the law and the evidence."
Khan said his office reserved the right to issue a new arrest warrant if more evidence came to light.
The ICC issued an arrest warrant for Mokom in 2018 and he was finally handed over by Chadian authorities last year.
Set up in 2002, the ICC is the world's only independent tribunal capable of prosecuting those accused of the world's worst crimes.
Anti-Balaka attacks continued on Muslim civilians even after Seleka forces retreated from Bangui, until at least December 2014.
Peace agreements signed in 2017 and 2019 have not been respected on the ground in CAR.
Many armed groups, rebels and assailants continue a guerrilla campaign of sporadic attacks against the Central African army, which is supported by mercenaries from the Russian private security company Wagner.
Both sides are regularly accused of crimes and abuses against civilians by international NGOs and experts mandated by the United Nations.
Two former anti-Balaka leaders, Patrice-Edouard Ngaissona and Alfred Yekatom, are already on trial at the ICC.
Last year Seleka commander Mahamat Said Abdel Kani denied war crimes and crimes against humanity charges before the court.