YEMEN (REUTERS) - The war in Yemen has raged for almost two years and according to the UN - killed more than 10,000 people.
Many who've died - are victims of attacks - led by Saudi Arabia, a big buyer of UK arms.
That - says the Campaign Against Arms Trade (CAAT) - is why it's taken the British government to court.
The NGO alleges British-made fighter jets and weapons have been used in the conflict in direct violation of UK and EU laws.
Spokesman for Campaign Against Arms Trade Andrew Smith said: "For two years now the UK has been complicit in the Saudi-led destruction of Yemen. We have seen UK fighter jets and UK bombs being central to that bombardment and central to the destruction which has taken place."
Britain is the fifth biggest arms exporter in the world.
One of its weapons makers BAE Systems has done multiple deals with Saudi Arabia in the past.
Last year the company said sales to the Gulf state were a key driver of its profit.
CAAT says deals like that must stop.
Smith added: "On paper UK arms export criteria is very clear. It says if there is a clear risk that weapons might be used in a serious violation of international humanitarian law then arms sales should not go ahead."
In November the British government said that's something it's well aware of and the UK's arms export trade is one of the most robust in the world.
Any similar arguments made in this three day court are unlikely to be heard by the public.
The government's defence is being heard in a closed hearing.