Microwave networks are harder to analyze than their lower-frequency counterparts. The reason is that the size of a typical microwave circuit is comparable the wavelength, so phase variation along a portion of the circuit cannot be ignored as is the case at lower frequencies.
The history of microwaves started with Maxwell’s theory in the nineteenth century. Maxwell mathematically showed that electromagnetic wave propagation exists, and that light is an electromagnetic wave. Not many people understood Maxwell’s theory at the time.