Answer: B. Wells wanted to illustrate that England could be caught with its guard down.
The War of the Worlds is a novel by H. G. Wells, first published in 1897 by Pearson's Magazine. It tells the story of the conflict between humans and a race of aliens.
The book is also famous because it was read aloud on a famous broadcast on October 30, 1938. An "urban myth" (as its veracity is disputed) says that when the broadcast aired over the Columbia Broadcasting System radio network, it caused panic among the listening audience, who thought the alien invasion was a piece of news.
The book uses Martians as the attackers because of the unexpected nature of an attack like that. Wells wanted to illustrate the consequences of being caught with your guard down.