Final answer:
General Butler warned Jefferson Davis about the importance of the lower Mississippi to Grant.
Step-by-step explanation:
The person who warned Jefferson Davis that the lower Mississippi was important to Grant was Butler. During the American Civil War, General Benjamin Butler, who was commanding the Union forces, captured New Orleans from the Confederates in 1862. This gave the Union control over the lower Mississippi River, which was crucial for their military strategy and the supply of resources.
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