How physics ended the nighttime radio war between the US and Mexico

The 1930s witnessed a technological war between the US and Mexico. Radio channels overlapped with each other, turning the nighttime radio world into bilingual chaos. How did it end? Physics! And diplomacy.

In the early 1900s, as radio stations gained popularity, the world became a different place. National stations rose up, letting people hear voices from distant regions on a regular basis. (Some believe that this shaped modern language and stamped out many regional accents, as people in different areas of a country began mimicking the voices they heard on the radio.) But the radio world didn't necessarily bring unity. Invisible factions competed for territory, trying to promote their own signals and drown out their competitors. Even a well-established station could get drowned out - or could be ruined by interference - from a local station that moved in on their frequency.
This was especially apparent along the border of the United States and Mexico. Americans found, often to their annoyance, that Mexican stations had the technological edge. "Border blaster" stations out-powered American stations for decades. They had the stronger signal. This was especially apparent at night. People tuned in to one station during the day, only to find at sunset that another station had taken over.
How physics ended the nighttime radio war between the US and Mexico
What happened? It turns out, there's such a thing called a skywave. This is an electromagnetic wave, generated by a radio station, that shoots up toward the sky and bounces off the ionosphere. It heads back down to the ground, providing a very long range for a station. Sometimes.
The bouncing can't happen all the time. During the day, the ionosphere is so perturbed by sunlight that radio signals get lost. So while the skywave is drowned out during the day, at night it comes through and people found their favorite radio stations taken over by other signals. (East coasters can enjoy the skywave during the morning hours, as it's still bouncing off the sky in the still-dark west. West coasters lose the skywave in the morning, but catch a pre-sunset skywave because it's already dark in the east.)

After decades of fighting, Canada, the United States, and Mexico got together to establish certain radio signals throughout the western hemisphere. They needed a little help from physics. They isolated which frequencies could maintain a skywave, and they called those frequencies "clear channel stations." The stations assigned to these frequencies would be required to maintain a certain level of power at all times, and lower-powered stations couldn't move in on these stations and disrupt a larger broadcast. Local, low-power stations were also assigned a certain set of frequencies, so their broadcasts wouldn't be overpowered by national stations.
On March 29, 1941, pretty much all the radio stations in the western hemisphere swapped places. And at last, there was a truce. Until podcasting came along.