"The US’s liability is evident on two grounds. Firstly, it is a party to the conflict by providing intelligence and it is providing arms to a state that is potentially responsible for grave human rights abuses. Since the start of the conflict, Saudi Arabia’s air strikes have caused widespread civilian casualties that have ignited calls for a UN inquiry. Secondly, the US is providing arms as a so-called "legal" transfer to a state that is liable for human rights abuses." (thanks Amir)