Why was lincoln being advised to free the slaves?
the north was losing the war and needed from the slaves.
the blockade of southern ports was ineffective.
this would prevent the recognition of the confederacy by england.

The answer is "this would prevent the recognition of the Confederacy by England." This is because at first, the war was about states rights, and if England ended up backing the South in the Civil War, the North knew they couldn't win. By making the Civil War about slavery, Lincoln pushed England out of the war. England had abolished slavery years before and would never support the Confederacy if they were fighting to keep their slaves.

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