Final answer:
Abraham Lincoln believed it was essential to retain the Border States in the Union during the Civil War due to military strategy, impact on the South's ability to fight, and diplomatic reasons.
Explanation:
Abraham Lincoln viewed keeping the Border States in the Union as crucial during the Civil War for several reasons. This included military strategy, the impact on the South's ability to fight, and the diplomatic implications of embracing emancipation by the Union, which would also allow for the recruitment of African American soldiers.
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