I've just seen this expression "useful idiots" and I know it is most often attributed to Lenin, the leader of the Bolshevik revolution establishing the Soviet Union.
People who use the expression these days usually refer to leftist anti-war types or anti-zionists etc. It suggests that Lenin referred to communists as useful idiots but I reckon if he said it at all and he was referring to people who helped or supported the revolution he can't have meant communists or leftists or he would have been calling himself an idiot.
I think I remember hearing the expression attributed to Lenin back when I was at university. I graduated 1982. I can't be bothered to research this much but I saw on Wikipedia that there is no evidence that Lenin said or wrote "useful idiots" anywhere.
Well I think I remember reading that Lenin did indeed say it. According to what I think I read, there was a famine in the fledgling Soviet Union. Western charities arranged for food aid. Philanthropy, according to Lenin, was all about palliative relief for the poor to undermine the long term relief brought by the revolution. So philanthropists who did their thing to undermine the revolution were alleviating famine in the Soviet Union thereby making themselves useful to the revolution.
Hence, useful idiots. No? Yes? Any takers?