When Federal forces first made a lodgment on Tybee Island, the work on Fort Pulaski was progressing slowly, but Robert E. Lee's judgment as the District's commanding general was that "the river cannot be forced". Old Fort Jackson had been armed, strengthened and "forms an interior barrier". Savannah's channel had been blocked. In December, Lee reasoned since the Federals had sunk a Stone Fleet in the Charleston Harbor, they did not intend to use it. "We must endeavor to be prepared against assaults elsewhere on the Southern coast." To that end, additional ships were sunk by Confederates in water approaches that led behind Fort Pulaski.