What did Secretary Seward mean when he said Lincoln should 'wait for the eagle of victory'?

To wait for the Union's victory before he declared the Emancipation Proclamation.

Explanation:

William Seward was the Secretary of State, under President Lincoln. The statement is an advice to President, to wait for the Union's victory before he declared his Emancipation Proclamation.

He said this because he believed that such declaration would not hold if the Union lost the civil war to the Confederates.

Hence, he advised to "wait for the eagle of victory" so that President Lincoln could outrightly issue his Emancipation Proclamation after the Confederates must have been defeated.

The Emancipation Proclamation was a document that declared all slaves within the Southern states shall be freed after the American Civil War.

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