What was James Clerk Maxwell's major accomplishment?

Answer 1

It was the new theory of electromagnetics that combined electricity and magnetism into one unified theory.

Albert Einstein wrote: "With the help of new field concepts Faraday formed a qualitative explanation of the whole concept of electromagnetics. The precise formulation of the time-space laws of those fields was the work of Maxwell. Imagine his feelings when the equations he had formulated proved that electromagnetic fields spread as waves, and with the speed of light! Few men in the world have enjoyed such an experience. At that thrilling moment he never guessed that the nature of light would continue to baffle succeeding generations. It took physicsts some decades to grasp the full significance of Maxwell's discovery, so bold was the leap that his genius forced upon the ideas of his fellow workers. Only after Hertz had demonstrated the existence of the waves experimentally did resistance to the new theory break down".