Over all of Spain, the sky is clear

To use the medical jargon, radio has some off-label uses.We listen to the radio for entertainment and information. We hear and interpret an audio message for the most part just as it is intended. But in some rare cases a single phrase is imbued with more meaning. I can think of two cases where a radio broadcast was used to trigger immediate military action. Only one is well known, well-documented and a part of the accepted historical cannon. One was in 1975 in Vietnam; you can read about it here.  The other was in Spain in 1936 and is apocryphal.

In American history, war is often taught and understood as a battle between good and evil and between two diametrically opposed sides. The truth is usually a lot messier, and the Spanish Civil War stands as a stark example. The short version is that it was fought from July of 1936 to the Spring of 1939. The primary sides were the Republicans, and the Nationalists. The Republicans were loyal to the ruling establishment (a post-monarchy elected government), and the Nationalists, led by General Franco. If you don't remember Franco, just know that he was a fascist dictator similar to Mussolini who sided with Hitler in WWII. The Nationalists won, and Franco ruled Spain for until his death 36 years later.  Other groups involved in the fighting the war included Socialists, Communists, the Basque, Catalonian separatists, anti-fascist Anarchists and even"Carlists" who favored power for a family of royal lineage. You can read more here.

So as the story goes,  a signal was broadcast by radio to trigger the Nationalist Military Revolt in Spain on July 17th 1936. The words were "Over all of Spain, the sky is clear" In Spanish this was probably rendered as "En toda España, el cielo está despejado."  The story is short and lacking in detail. In 1936, broadcastingn was only 13 years old in Spain and there was no municipal or government owned radio networks. Most texts put Spain as having fewer than 60 stations total at that time, most low powered and clustered around major cities like Madrid and Barcelona. Most historical sources claim this was broadcast on"numerous" radio stations. The places where the "Falange" was successful were rural and highly Catholic provinces. It is probable that the signal was carried on private or amateur stations, in towns where later "autorizadas" operated semi-officially in support of Franco. At the start of the war Franco was in the Canary Islands so it's likely there was a broadcast there as well.

After the revolt began, the government responded by closing all low powered stations, and suspending all amateur radio activity. All radio official news broadcasts were carried on Radio Union in Madrid and Barcelona and the shortwave station EAQ in Aranjuez. Loyalists also controlled EAJ2 and EAJ at Barcelona, though at times they just broadcast an empty carrier wave to prevent the rebels from using those frequencies. Franco already held Radio de Castilla Burgos and was seizing amateur equipment to broadcast propaganda as well.

In 1937,  the municipal station in RNE (Radio Nacional de España) was founded in Salamanca. It's first transmitter was a 20 kW Telefunken and was a gift from Nazi Germany to the Franco government. After the war ended it became the primary source of propaganda all other stations mandated to carry it's daily news bulletins. After Franco died in 1975 censorship was curtailed, and democracy came to Spain and the bulletins ceased and RNE slowly re-made itself into a more BBC-like entity.