The microwave was invented accidentally in 1945 by a self-taught engineer named Percy Spencer, who was leading a radar project for the defence giant, Raytheon. While testing a new vacuum tube called a magnetron, he discovered that a chocolate bar in his pocket had melted from the heat. He decided to try another experiment by placing some popcorn kernels near the magnetron, and he watched as the kernels popped into fluffy popcorns.

[https://celcook.ca/the-accidental-invention-of-the-microwave/]

I believe this reaserch was because they were trying to find alternative ways to communicate during WW2.

Engineers planned to build a new radar system based on electromagnetic waves in the microwave region of the radio spectrum. Such a system would require smaller antennas and detect smaller objects than lower-frequency, longer-wavelength radars.