What does The Martian Chronicles have to do with American history?
The Martian Chronicles can be thought of as commentary on Westward Expansion. America has always had a frontier; usually it was the West, and when Bradbury was writing in the late-1940s, the next frontier seemed to be space. The existence of a frontier is tied up in the impulse to fill it up with civilization, with rugged individualism, and with the "American Dream." That is, the man who goes to the frontier finds not only rugged adventure, but also a chance to improve his socio-economic standing. Bradbury shows the contradictions inherent in this idea. First, while the frontier should be romantic and inspiring, the act of settling involves marring the landscape. This is seen in Spender's attempt to save the landscape, in Parkhill's garish hot dog stand, and in numerous other details. Second, the brave settlers may be heroic, but to native civilizations they can seem like the end of the world. Bradbury hints at this in the character of Cheroke, whose ancestors were Native Americans.
Contrast two settlers' views of Mars.
LaFarge and the old man at the gas station have very different views of life on Mars. The Martian who visits LaFarge looks like his deceased son, Tom, because that is what LaFarge is looking for. When that Martian goes running through town, many people see many different people in him, but no one sees a Martian. This is because no one wants to see a Martian. Each person wants something very specific from Mars, and it is not a native Martian. They don't want to experience something different; they want to experience something familiar. This outlook is shared by many settlers, as Bradbury often mentions that the hastily-constructed villages all resemble American towns. The old man with whom Gomez talks at the station has a very different view. He expects Mars to be different. He is pleased by the wonderful variety and originality of the planet, and he compares living there to looking through a telescope.
What does the story "Usher II" have to do with the rest of the novel, other than the fact that it is set on Mars?
Of course, The Martian Chronicles is a very fragmentary book, and "Usher II" was not necessarily written to be a part of it. Many of the stories in the novel were written before Bradbury decided to combine them. Nonetheless, "Usher II" shares an important theme with the rest of the novel. Stendahl comes to Mars looking for an escape from the bureaucracy and censorship that he faced on Earth. In other words, he is looking for a wilderness. He is fleeing civilization. He is frustrated that the Investigators of Moral Climates have already found their way onto Mars. He wishes that Mars could be left alone. He is like a Martian, sorry to see the Americans come and try to replicate America on Mars. He is also like Spender in this respect. Although they all have different motives, they all resist the waves of civilization that wash over Mars.