To hear the Federal Bureau of Investigation tell it, its decision to launch a counterintelligence probe into a major-party presidential campaign comes down to a foreign tip about a 28-year-old fourth-tier Trump adviser, George Papadopoulos.

The FBI’s media scribes have dutifully reported the bare facts of that “intel.” We are told the infamous tip came from Alexander Downer, at the time the Australian ambassador to the U.K. Mr. Downer invited Mr. Papadopoulos for a drink in early May 2016, where the aide told the ambassador...