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

Seward had always advocated for the freeing of all slaves, but with this proclamation he felt that "such a proclamation ought to be 'borne on the bayonets of an advancing army, not dragged in the dust behind a retreating one." Seward's suggestion to wait until after a victory was adopted along with his other: that