We’ve Known About the Threat for Awhile
During the U.S.’s first nuclear test in July 1945, electronic equipment was shielded due to expectation of an EMP resulting from the detonation. The official history for that first test states, "All signal lines were completely shielded, in many cases doubly shielded. In spite of this, many records were lost because of spurious pickup at the time of the explosion that paralyzed the recording equipment."
In the summer of 1962, the U.S. tested a 1.44 megaton nuclear device in space, 250 miles above the Pacific Ocean near Johnson Island. The test, named Starfish Prime, proved to scientists that effects of a high-altitude nuclear explosion were much larger than had been previously calculated. It was hard to keep these facts a secret because the test caused electrical damage some 900 miles from ground zero in Hawaii. Reportedly, about 300 streetlights went out, burglar alarms went off, and a telephone company microwave link was damaged.
Note that if the Starfish Prime warhead had been detonated over the northern continental United States (rather than the lower latitudes near Hawaii), the magnitude of the EMP would have been much larger. This is attributable to the greater strength of the Earth's magnetic field over the United States, as well as an orientation of the Earth's magnetic field at high latitudes that is more favorable to the EMP effect.
OK, so 300 streetlights does not sound like much. Consider the fact, however, that these effects manifested 900 miles from the test. Also, consider the electronics infrastructure of 1962 was far more robust than what is in place today[md]in effect, the equivalent of a wind-up alarm clock in comparison to a digital watch. There was scarcely a transistor to be found at that time because solid state technology was new. Vacuum tubes, on the other hand, are thousands of times more resistant to EMP. Although the transistor was invented in the mid 1950s, it was not until the early 70s with the advent of the silicon chip that the potential impact of EMP became cause for concern.
Soviet Cold War–era military aircraft often had avionics based on vacuum tubes due in part to limitations in Soviet solid-state capabilities at the time, but also based on the proposition that vacuum-tube gear would survive better. In 1979, I actually served in Japan when a Soviet MIG-17 defected and turned up in Hokkaido. Given a rare glimpse at this mysterious Soviet fighter plane, U.S. and Japanese authorities chuckled at the arcane vacuum tubes in the radio and other equipment. I had suspected for years that the technology was used deliberately, because a shooting war at the time could involve tactical nuclear weapons on both sides, and a resultant EMP. Guess I was right.