A Useful idiot designates a naive or credulous person who can be manipulated or exploited to advance a cause or political agenda.

A useful idiot is described as someone who supports one side of an ideological debate and at the same time is manipulated while unaware of the ultimate agenda driving an ideology they have subscribed themselves to.

“It’s a term that refers to brainwashed American Marxists who blindly support any ideology that gets themselves out of real work and causes others to pay their way.” 

Useful idiots, in a broader sense, refers to Western journalists, travelers and intellectuals who gave their blessing — often with evangelistic fervor — to tyrannies and tyrants, thereby convincing politicians and public that utopias rather than Belsens thrived.