International law explained: What are genocide, war crimes and crimes against humanity?
Editor's note: This is one of four explainers about international law. The series also includes separate pieces about war crimes, genocide, and crimes against humanity.
Israel and Hamas have been accused of serious international crimes as a result of Israel’s war on Gaza, during which at least 48,440 Palestinians have been killed as of 5 March 2025.
Israeli Prime Minister Benjamin Netanyahu declared war on 7 October 2023, following a Hamas attack on southern Israel, which killed 1,195 people and saw a further 250 taken captive. A ceasefire came into effect on 19 January.
In 2024, Israel stood accused before the International Court of Justice (ICJ) of allegedly violating the Genocide Convention. And the International Criminal Court (ICC) has issued arrest warrants for Netanyahu and ex-Defence Minister Yoav Gallant. Warrants for Hamas military chief Mohamed Deif, leader Yahya Sinwar, and Ismail Haniyeh, head of its political wing, have been withdrawn after all three were killed.
International courts, rights groups, UN experts, and agencies have alleged that genocide, crimes against humanity, and war crimes have been committed during the conflict.
But what is the difference between each of the major international crimes? How has the system of international law evolved? And who decides whether the accused are guilty?
The Rome Statute of 1998 is a foundation of international criminal law. It refers to the four major crimes - genocide, crimes against humanity, war crimes, and the crime of aggression - as “the most serious crimes of concern to the international community as a whole”. Scholars refer to the first three of these collectively as “core international crimes” or “atrocity crimes".
Such crimes are usually committed during armed conflicts or as part of government policy: their severity and scale are far greater than other serious crimes.
Often they cannot be tried in the country where they were committed, hence the need for an international justice system.
There is much overlap between each class of crime - but the broad focus of each is as follows:
Genocide: The destruction, or intent to destroy, a national, ethnic, racial, or religious group
Crimes against humanity: An act against civilians as part of a wider action
War crime: When the rules that govern conflict and protect non-combatants, such as civilians, are broken
The crime of aggression: The use of force by a state against another state
The definition of what are international crimes has emerged in response to conflicts during the past two centuries.
War crimes were formally introduced with the Lieber Code during the American Civil War, issued to Union forces in 1863. Troops were prohibited by their leaders from carrying out acts of torture and cruelty. There were also rules to govern the treatment of civilians, injured soldiers, prisoners of war, and property.
The code went on to shape international legal frameworks, especially the Hague Conventions of 1899 and 1907, which were among the first major international treaties aimed at regulating war.
Raphael Lemkin, a Jewish Polish lawyer, devised the word "genocide" in 1944, in the wake of the Holocaust perpetrated by Nazi Germany, to describe the systemic murder of European Jews and other groups.
Unlike war crimes and genocide, there are no treaties dedicated to crimes against humanity or the crime of aggression. The International Law Commission, the UN body that develops international law, is currently working on a draft convention for crimes against humanity. At present, Article 7 of the Rome Statute provides the most detailed definition.
No. There has been much discussion about ethnic cleansing since the start of Israel’s war on Gaza and its attempts to drive Palestinians from the territory. In February 2025, US President Donald Trump was condemned for supporting ethnic cleansing in his plan to displace Palestinians from Gaza.
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") rgba(220, 220, 220, 0.5); top: -15px; left: 0px;">The term ethnic cleansing first emerged during the Yugoslav wars of the 1990s. It describes the forced displacement of an indigenous national, ethnic, racial, or religious group with the intent of making the area ethnically homogenous.
The phrase often comes up in discussions about serious international crimes. And while ethnic cleansing is not defined as a crime itself, its underlying intent is reflected in Article 49 of Geneva Convention IV which mentions “forced transfers” and "forced deportations” as war crimes. Likewise, the Rome Statute bans forced transfers of civilians and deportations, describing them as crimes against humanity and war crimes.
Broadly speaking, to prove any crime, courts must establish two elements: (i) that a criminal act was committed; (ii) it was done with intent or knowledge.
Courts also establish how an individual is legally responsible for the act. For example, did the accused carry it out directly themselves? Or was it indirect: were they, for example, a commander or superior responsible for subordinates who committed a crime?
International crimes can be prosecuted by international courts, as well as by one-off, or ad hoc, tribunals. National courts can also prosecute core international crimes under universal jurisdiction, which allows a country to pursue a case, regardless of where the crime was committed, or the nationality of the victim or the alleged perpetrator.
The key international judicial bodies at present are the International Court of Justice (ICJ) and the International Criminal Court (ICC). Much of their work overlaps: they both sit in The Hague in the Netherlands, for example. But each serves a distinctive purpose.
") rgba(220, 220, 220, 0.5); top: -15px; left: 0px;">Origins: The ICJ was founded in 1945 as the principal judicial organ of the UN. The ICC began its work in 2002 after the Rome Statute of 1998.
Who does what? The ICJ intervenes in disputes between states, including interpretations of human rights treaties; rules on breaches of international law; and reparations. The ICC looks at cases against individuals when national judiciaries are unable or unwilling to do so.
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") rgba(220, 220, 220, 0.5); top: -15px; left: 0px;">Structure: The ICJ has 15 judges, nominated by countries that signed the court's statute, and elected for nine-year terms by the UN General Assembly and UN Security Council (UNSC).
The ICC has 18 judges, appointed for nine years and serving in different chambers. They are nominated by states, which signed the Rome Statute, and are then elected by the Assembly of States Parties, the court's governing body.
Membership: The ICJ can be used by any of the UN’s 193 member states. The ICC has 125 signatories, including most of Europe. But it is not recognised by the US, Israel, Russia, China, India, and much of the Middle East and North Africa (MENA).
During the 20th century, tribunals were established to deal with specific conflicts. It is from these that much of the international justice system evolved.
The first significant attempt to prosecute war crimes came after World War I, when the Treaty of Versailles (1919), drawn up by the victorious Allied powers including the US, France, and UK, proposed trials for the defeated German leaders.
The effort largely failed - but the list of 32 war crimes set a precedent for stating that war crimes were prohibited. Two courts, created after the end of World War Two in 1945, became hugely influential in this process.
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") rgba(220, 220, 220, 0.5); top: -15px; left: 0px;">The first court was the International Military Tribunal or IMT, which held the Nuremberg Trials. It sat in the central German city, and prosecuted Nazi leaders for crimes against peace, war crimes, and crimes against humanity. Founded through the London Charter of 8 August 1945 by the Allied powers (the US, UK, USSR, and France), the IMT ran from 20 November 1945 to 1 October 1946. Its verdicts included death sentences on 12 Nazis, among them Foreign Minister Joachim von Ribbentrop and Interior Minister Wilhelm Frick.
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") rgba(220, 220, 220, 0.5); top: -15px; left: 0px;">The second court was the International Military Tribunal for the Far East, better known as the Tokyo Trials, which sat in the Japanese capital and prosecuted wartime leaders. Taking place under the auspices of General Douglas MacArthur, who was in charge of post-war Japan on behalf of the Allies, the tribunals, which ran from 1946 to 1948, sentenced seven men to death, including one-time Japanese prime minister Hideki Tojo.
Both trials pioneered major legal principles. The most important was that of individual criminal responsibility: no person, including a head of state, could now defend themselves by saying that they were only following orders or acting in an official capacity.
During the 1990s, the United Nations Security Council established two specialised tribunals whose judgments were to be significant for the development of international criminal law and the ICC.
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The first was the International Criminal Tribunal for the Former Yugoslavia or ICTY. It was formed in May 1993 amid the widespread conflict of the Yugoslav Wars (1991–2001), during which at least 140,000 people were killed. Based in The Hague, it prosecuted individuals responsible for atrocities in Bosnia, Croatia, Kosovo, and Serbia. The ICTY issued 90 sentences during its 24 years, including life imprisonment for the former Bosnian Serb leader Radovan Karadzic and General Ratko Mladic.
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") rgba(220, 220, 220, 0.5); top: -15px; left: 0px;">The second was the International Criminal Tribunal for Rwanda or ICTR. It was formed in November 1994 by the UN Security Council amid the Rwandan Genocide (April–July 1994), during which approximately 800,000 Tutsi and moderate Hutu were killed over three months. Based in Arusha, Tanzania, the ICTR primarily targeted leaders of the Hutu-led Rwandan government and military.
The tribunal issued 61 sentences including life sentences against former Rwanda Prime Minister Jean Kambanda and former Family Minister Pauline Nyiramasuhuko - the first woman convicted of genocide. The ICTR closed in 2015.
Both the ICTY and ICTR focused on cases in the former Yugoslavia and Rwanda respectively. The Rome Statute established the ICC: the first permanent institution to hear international crimes against individuals from anywhere.
The court began its work in 2002: to date, it has heard 32 cases and imprisoned 21 individuals. An estimated 125 countries are signatories to the Rome Statute - around two-thirds of the world’s nations. Regionally, there are big discrepancies.
The Americas: The ICC is recognised by most of North and South America, including Canada and Mexico. The US was an original signatory but then withdrew amid fears of implications that soldiers would be held accountable for actions in Iraq, Afghanistan, and elsewhere.
Europe: Most of the continent is a signatory: the major exception is Russia, an original signatory which, like the US, subsequently withdrew.
Middle East: Only Jordan, Tunisia, and Palestine are signatories to the statute. The rest of the region, including Turkey and Saudi Arabia, have yet to sign.
Asia and Oceania: Most regional powers including India, China, the Philippines, and Indonesia are not signatories to the Rome Statute. Those who recognise the court include Japan, South Korea, Australia, and New Zealand.
Africa: Much of central and southern Africa recognises the court, including Nigeria and South Africa, as well as Kenya, Tanzania, and Uganda.
This patchwork of countries matters hugely to anyone subject to a warrant from the ICC: should they enter a state that is a signatory to the statute, then that country's courts are legally obliged to hand the individual over to the ICC.
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") rgba(220, 220, 220, 0.5); top: -15px; left: 0px;">Take the example of Benjamin Netanyahu, who is currently subject to an ICC arrest warrant. The US is not a signatory: Netanyahu therefore did not risk extradition when he visited Washington in January 2025.
But Netanyahu would need to take more care in western Europe, where several allies of Israel, including the UK and Germany, are signatories.