Lincoln declared a federal blockade of all southern ports as early as May 1861, but the Union Navy took some time getting ready for that task. By August, the Union Navy made attacks against the Confederate forts at Cape Hatteras, and by the early autumn, it was in command of that vital sea-link. Here, too, the importance of Fort Monroe was again evidenced. If the Confederacy had been able to take Monroe, it would have had a clear path to the Atlantic Ocean. Lacking that, the Confederates had to ship materials down to the North Carolina coast and out the outlets there.

The North had a major, even a huge, advantage at sea, with hundreds of vessels compared to just a few for the Confederacy. Lincoln and his advisers had one great, gnawing concern, however: what if either Britain or France threw its weight behind the Confederacy? It was conceivable that the Northern blockade would be smashed and that Confederate cotton would flow to Europe, earning the Confederacy both dollars and an increased reputation.