Answer :
Final answer:
During Reconstruction, the army in the South first registered voters to uphold rights granted by the First Enforcement Act of 1870.
Explanation:
Once the military districts were established in the South during Reconstruction, the first thing the army did was register voters. This process aimed to enforce the rights granted by the First Enforcement Act of 1870, which made it a federal crime to interfere with voting rights and allowed the president to use troops to maintain order at the polls.
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