Wordorigins.org

The phrase the tail that wags the dog dates to the late 19th century. The meaning is quite obvious, the subsidiary part is controlling the major part. In current usage, it is often applied to idea the media creating a crisis instead of a crisis generating media interest. It came to recent prominence in the 1997 film Wag the Dog starring Robert DeNiro and Dustin Hoffman.

The phrase appears in Kipling’s 1892 The Conundrum of the Workshops:

We know that the tail must wag the dog, for,
the horse is drawn by the cart.

(Sources: Yale Book of Quotations; Oxford English Dictionary, 2nd Edition.)