Useful Idiot

Someone who promotes some cause, but doesn’t totally understand it, or is ignorant of the fact that the cause is actually working against their own interests. They are an “idiot,” but a “useful” one, as far as the cause is concerned.

No one is sure of attribution. Many people attribute it to Lenin, but an investigation by a NY Times columnist in 1987 found no evidence of this.

From that article:

Referring with derision to what he called “so-called notables who were convened as props for Gorbachev’s speech,” Zbigniew Brzezinski used a stinging phrase that has become a term of art among hard-line historians and former national security advisers: useful idiots of the West.