red herring

(figurative): One possible origin of the idiom was that red herring were used to train dogs to track scents. This was true, but the modern meaning of a false trail may have been popularized in a news story by English journalist William Cobbett, c. 1805, in which he claimed that as a boy he used a red herring (a cured and salted herring) to mislead hounds following a trail; the story served as an extended metaphor for the London press, which had earned Cobbett's ire by publishing false news accounts regarding Napoleon. The OED has another possible earlier origin in the legacy of clergyman Jasper Mayne in 1672 when he misled a servant by leaving him "Somewhat that would make him Drink after his Death" in a large trunk. When the trunk was opened, the contents were found to be red herring.

(soldier): An allusion to soldiers' red uniforms; soldier is, reciprocally, a slang term for the fish.