South Africa's case against Israel over allegations of genocide before the international court of justice has raised a central question of international law: what is genocide and how do you prove it? It is one of three genocide cases being considered by the UN's world court, but since the genocide convention was approved in 1948, only three instances have been legally recognised as genocide. Josh Toussaint-Strauss looks back on these historical cases to find out why the crime is so much harder to prove than other atrocities, and what bearing this has on South Africa's case against Israel and future cases

• What is the genocide convention and how might it apply to the UK and Israel?

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