It's been 20 years since Europe's worst atrocity since World War II

With the support of Yugoslav president Slobadon Milosovic's government in Belgrade, the Bosnian Serbs - under the leadership of Radovan Karadzic, who is now facing a range of war crimes charges  - were attempting to liquidate Bosnia's Muslim population as part of an attempt to carve a "greater Serbia" out of the ruins of Yugoslavia, the polyglot communist state whose breakup into 7 different countries began in the early 1990s. Bosnia has sizable Muslim, Croat, and Serbian populations, and was the one republic of Yugoslavia without a clear ethnic majority (see map at left).

Milosovic, Karadzic, and Bosnian Serb militants under the leadership of Ratko Mladic used ethnic cleansing to cleave off as much of Bosnia as possible for the Serbian-dominated remains of Yugoslavia.

The Srbrenica massacre was the inevitable result: an act of mass-murder that conveyed the brutal message that Muslims weren't safe anywhere inside of the country - and that the UN and the international community were either unable or unwilling to protect them.

In 1993, the UN had established a demilitarized zone in Srebrenica, creating an area where Muslims who had been forced out of their homes elsewhere in Bosnia could find safety from the Bosnian Serb onslaught.

Bosnian Muslim militants allied with the govenrment in Sarajevo had carved out an enclave in eastern Bosnia surrounding the Srebrenica safe-zone. The Serbs wanted to take this pocket of resistence, and their military leaders resented the UN providing shelter for displaced Muslims. A declassified CIA memo from the time described the handful of UN eastern safe-zones as "fish bones in the throat of the Serbs."

The massacre had been planned in advance. The week of the atrocity, Serbian forces had taken surrounding villages, forcing some 20,000 refugees inside the UN safe-area. Serbian forces had also kidnapped 30 Dutch peacekeepers, a blunt instrument of blackmail and leverage over the Dutch peacekeeping force guarding the enclave. And they had started shelling Srebrenica on July 6th, making it abundantly clear that they would not respect the UN's humanitarian safe area.

In the hours leading up to the killing, Mladic, who is also currently facing war crimes charges, can be seen on video laughing and handing out candy to the troops in what veteran CNN reporter Christiane Amanpour describes as "one of the most chilling pieces of video I've ever seen in my life."

The Serb forces were greatly aided by the international community's indifference. Though the UN wanted a peacekeeping deployment of around 6,000 in the area, by the time of the massacre only around 600 lightly-armed Dutch troops were guarding the town. When Maldic and the Bosnian Serb army entered Srebrenica, the peacekeepers put up little resistance and even called off airstrikes when the Serbs threatened to kill their Dutch hostages. Peacekeepers were also later accused of destroying video evidence of their inaction.

ReutersRefugee women read the names of missing men written on pillows July 18, 1995 at the site near the Bosnian town of Kladanj where Moslems from Srebrenica crossed the former front line three years earlier, escaping from Bosnian Serb forces. Srebrenica, established as the UN "safe haven" was overrun by the Serb forces in one of the worst war atrocities since 1945 on July 11, 1995.

Far from protecting vulnerable civilians, the "safe zone" had just concentrated them in a single location that the UN apparently had little intention of actually defending. But they weren't the only party at fault: as a Human Rights Watch report from late 1995 recounts, the NATO states remained complacent and indecisive even as the enclave's fall was imminent - and despite the clearly genocidal intentions of the Serbian forces.

ReutersAround 10.000 refugees from Srebrenica board buses at a camp outside the UN base at Tuzla Airport heading for other refugee camps in the Tuzla area on July 14, 1995.Serb forces started deporting all women and children from the enclave as soon as Srebrenica fell on July 11th, and held nearly all of the area's Muslim males for "interrogation." More than 8,000 of them would be killed in the following days.

The accords ended the conflict. But they led to an internal partitioning of Bosnia while arguably awarding Serb forces for their atrocities.

ReutersA Bosnian Serb soldier fires a heavy machine-gun as his comrade holds the cartridge belt during "mopping-up operation" near the eastern Bosnian town of Srebrenica on July 13, 1995.Sreberenica played an outsized role in bringing about this indecisive end to the conflict. And the atrocity was on such a massive scale that victims are still being disinterred from mass graves in the area and identified.

Each year on the anniversary of the killing, the Bosnian government releases bodies that were recently discovered, in whole or in part, in the hills and fields that surround the town. The friends and relatives of the victims attend a mass funeral each year. 

On May 26, 2011, Gen. Ratoko Mladic was arrested and detained in Serbia as a suspect in the genocide at Srebrenica. This past March, another eight soldiers were arrested on suspicion of having participated in the killing.

The massacre is a still-open wound for the UN, and arguably the entire international system that it represents. In July of 2014, a Dutch court found peacekeepers responsible for the deaths of 300 people at Srebrenica, resolving one of several lawsuits connected to the massacre. Recent research based on declassified CIA cables alleges that Britain and US knew for 6 weeks that Srebrenica was close to falling to Serbian forces, but decided not to intervene out of concern that the crisis would get in the way of ongoing peace negotiations.

The slow western response, and the failure of the UN to prevent one of the worst single atrocities anywhere on earth after World War II despite the presence of their peacekeepers, raises serious questions about how outside actors should intervention in regional conflicts. Srebrenica exposed the US, NATO and the UN's fatal disorganization and indecision, and raised still-troubling questions about what the world's responsibilities should be when thousands of lives are in imminent danger.

What does it take for world powers to step in and stop human rights abusers before they can commit Srebrenica-like atrocities? When should international actors step in? if they do decide to intervene, what moral and political responsibilities do they assume? Is a country ever obligated to rescue people when it doesn't have a clear strategic or political interest in doing so, and should the moral imperative of protecting innocents ever override all other concerns?

Even 20 years after Srebrenica, there's still no clear answer.