In the first paragraph of Passage 1, the main purpose of Douglas's discussion of the growth of the territory and population of the United States is to:
Based on the given question, we can see that from the complete text, there is the narration by Douglass that the growth and territorial reach of the United States is important.
With this in mind, we can see that he made this statement so that he can prove that the free and ex slaves were not a danger to the Union.
Therefore, the correct answer is option D
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