For most of history, rulers who ordered mass killing faced no court at all. The twentieth century slowly built institutions to change that, and a fierce debate about when outsiders may intervene to stop atrocities.

Nuremberg

After the Second World War, the United States, Britain, the Soviet Union and France set up the International Military Tribunal at Nuremberg (1945 to 1946), which tried about two dozen senior Nazi leaders for crimes against peace, war crimes and crimes against humanity. A similar tribunal sat in Tokyo. Nuremberg established principles that still shape international law: individuals, not only states, can be guilty of international crimes; high office is no shield; and following superior orders does not free a person from responsibility, though it may reduce punishment. Critics called it “victors' justice” because crimes committed by the Allies were not tried, and some questioned charges that had not clearly existed in law before the war. Supporters answered that a fair trial, with evidence and defense lawyers, was far better than revenge or silence.

The 1990s tribunals

During the Cold War there was little international enforcement. Then, in response to the wars in Yugoslavia and the genocide in Rwanda, the UN Security Council created the International Criminal Tribunal for the former Yugoslavia (ICTY, 1993, in The Hague) and the International Criminal Tribunal for Rwanda (ICTR, 1994, in Arusha, Tanzania). In 1998 the ICTR's verdict against Jean-Paul Akayesu, a former Rwandan mayor, was the first conviction for genocide by an international court, and it also recognized that rape could be an act of genocide. The ICTY convicted senior commanders and politicians, including for the genocide at Srebrenica.

The International Criminal Court

States adopted the Rome Statute in 1998, and the International Criminal Court began operating in 2002 as the first permanent court for genocide, crimes against humanity and war crimes. It works on the principle of complementarity: it acts only when national courts are unwilling or unable to prosecute genocide, crimes against humanity and war crimes genuinely. More than 120 states have joined, but several powerful states, including the United States, China, Russia and India, have not. The court has no police of its own and depends on states to make arrests. Critics have said its early cases focused too heavily on Africa and that it moves slowly; supporters point to its convictions and to the value of a standing court that leaders know exists.

Intervention and sovereignty

Courts act after the fact. The harder question is whether and how outsiders should act while atrocities are happening. Sovereignty, the principle that states run their own affairs, protects weaker countries from domination by stronger ones. But it can also shield governments that attack their own people. The failures in Rwanda and Srebrenica, and NATO's 1999 bombing campaign over Kosovo without Security Council approval, sharpened the debate.

In 2005, at the UN World Summit, states endorsed the Responsibility to Protect (often shortened to R2P), with three pillars:

1. Each state is responsible for protecting its population from genocide, war crimes, ethnic cleansing and crimes against humanity.
2. Other states should help it meet that responsibility.
3. If a state clearly fails, the international community should be ready to take collective action through the Security Council, using peaceful means first and force only as a last resort.

The debate continues. Supporters see R2P as an overdue answer to the cry of “never again”. Critics worry that powerful states may use humanitarian language to pursue their own interests, as some argued happened after the 2011 intervention in Libya, and note that vetoes in the Security Council can block action in exactly the cases that matter most.

Worked example: Srebrenica from crime to verdict

Following one case through the justice system shows both what international justice can achieve and how slowly it moves.

• 1995. The killings take place in July. Within months, aerial photographs and survivor accounts point to mass graves.
• Late 1990s and 2000s. Investigators exhume graves, many of them disturbed when remains were moved to hide the crime. The International Commission on Missing Persons uses DNA testing to identify victims from scattered remains; over the years, the large majority of the missing have been identified and returned to their families for burial.
• 2004. The ICTY's appeals chamber confirms that the killings at Srebrenica were genocide, in the case of a Bosnian Serb general.
• 2007. The International Court of Justice, which hears cases between states, rules that genocide occurred and that Serbia violated its duty to prevent it, though it finds Serbia not directly responsible for committing it.
• 2008 and 2011. Radovan Karadžić and Ratko Mladić, after years as fugitives, are arrested and sent to The Hague.
• 2016 to 2021. Both are convicted of genocide and other crimes, and their convictions are upheld on appeal, with sentences of life imprisonment.

The process took more than a quarter of a century. Critics point to that delay and to its cost. Supporters point to what it produced: an exhaustively tested record of facts, the identification of thousands of victims, and verdicts that make denial far harder to sustain. Justice did not prevent Srebrenica, but it changed how the crime is remembered.