All parties recognized from the start that the blockading ships would have to be powered by steam. The limited endurance of steamships then implied that one of the first requirements would be possession of a harbor that would serve as a coaling station near the southern end of the blockading line, as otherwise blockaders would spend too much of their time going to and from home port seeking replenishment. All suitable harbors south of the Chesapeake Bay, however, were held by seceded states. In order to establish the blockade, therefore, at least one of them would have to be taken by the Federal forces. Thus, the blockade was immediately transformed from a purely open-water operation to one of at least limited occupation of enemy territory.