In the conclusion, the repetition of "it was" helps the author make his point in the sense that:
B. It emphasizes the many escalating events influenced by American exceptionalism.
Each of the paragraphs beginning with "it was" provides a more serious example than the one before of the ways in which exceptionalism served as a driving force.
The author is thus showing how events connected to that ideology escalated (became more intense), going from an idea applied to a defensive stance to an idea applied to intruding actions.
By repeating "it was", the author makes the listing of events more emphatic, more evident.
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