In some cases radio stations are considered legal where the signal is transmitted, but illegal where the signals are received—especially when the signals cross a national boundary. In other cases, a broadcast may be considered "pirate" due to the nature of its content, its transmission format (especially a failure to transmit a station identification according to regulations), or the transmit power (wattage) of the station, even if the transmission is not technically illegal (such as an amateur radio transmission). Pirate radio is sometimes called bootleg radio (a term especially associated with two-way radio), clandestine radio (associated with heavily politically motivated operations) or free radio.
Radio "piracy" began with the advent of regulations of the public airwaves in the United States at the dawn of the age of radio. Initially, radio, or wireless as it was more commonly called, was an open field of hobbyists and early inventors and experimenters. The United States Navy began using radio for time signals and weather reports on the east coast of the United States in the 1890s. Before the advent of valve (vacuum tube) technology, early radio enthusiasts used noisy spark-gap transmitters, such as the first spark-gap modulation technology pioneered by the first real audio (rather than telegraph code) radio broadcaster, Charles D. Herrold, in San Jose, California, or the Ruhmkorff coil used by almost all early experimenters. The navy soon began complaining to a sympathetic press that amateurs were disrupting naval transmissions. The May 25, 1907, edition of Electrical World in an article called "Wireless and Lawless" reported authorities were unable to prevent an amateur from interfering with the operation of a government station at the Washington, D.C. Navy Yard using legal means.
In the run-up to the London Radiotelegraph Convention in 1912 (essentially an international gentlemen's agreement on use of the radio band, non-binding and, on the high seas, completely null), and amid concerns about the safety of marine radio following the sinking of the RMS Titanic on April 15 of that year (although there were never allegations of radio interference in that event), the New York Herald of April 17, 1912, headlined President William Howard Taft's initiative to regulate the public airwaves in an article titled "President Moves to Stop Mob Rule of Wireless."
When the "Act to Regulate Radio Communication" was passed on August 13, 1912, amateurs and experimenters were not banned from broadcasting; rather, amateurs were assigned their own frequency spectrum, and licensing and call-signs were introduced. By regulating the public airwaves, President Taft thus created the legal space for illicit broadcasts to take place. An entire federal agency, the Federal Radio Commission, was formed in 1927 and succeeded in 1934 by the Federal Communications Commission. These agencies would enforce rules on call-signs, assigned frequencies, licensing and acceptable content for broadcast.
The Radio Act of 1912 gave the president legal permission to shut down radio stations "in time of war", and during the first two and a half years of World War I, before US entry, President Wilson tasked the US Navy with monitoring US radio stations, nominally to "ensure neutrality." The navy used this authority to shut down amateur radio in the western part of the US (the US was divided into two civilian radio "districts" with corresponding call-signs, beginning with "K" in the west and "W" in the east, in the regulatory measures; the navy was assigned call-signs beginning with "N"). When Wilson declared war on Germany on April 6, 1917, he also issued an executive order closing most radio stations not needed by the US government. The navy took it a step further and declared it was illegal to listen to radio or possess a receiver or transmitter in the US, but there were doubts they had the authority to issue such an order even in war time. The ban on radio was lifted in the US in late 1919.