Here's the quote with missing spaces inserted:
Many Americans, especially conservatives, cling to outdated or strangely distorted concepts of theirs as a frontier society or of what their founding fathers stood for; and the British to a peculiar sense of innate national superiority based on geographic separation from the rest of Europe, a lingering imperial nostalgia and a victory in the Second World War that would have been unachievable without massive US and Soviet involvement.
This is a long, challenging sentence. Try breaking it down into shorter bits and removing parenthetical clauses and some of the adjectives:
Many Americans cling to outdated or strangely distorted concepts of theirs as a frontier society. ("Theirs" means "their society"; "frontier society" refers to the effects that expanding into the Western frontier had on American culture.)
The clause after the semicolon refers back to the first clause. Where it says ...and the British to a peculiar sense of innate national superiority, it means The British cling to a sense of superiority. The following clauses list two more things the author thinks give the British a sense of superiority: imperial nostalgia (nostalgia for their lost empire) and their victory in WWII.
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