In the letter he suggested that it might be used to cause significant damage to a port. He had no idea that it would be (1) anywhere near as destructive as it was, or (2) small enough or light enough to drop from aircraft rather than only being used in either naval warfare (where death tolls would be much less) or as a fixed weapon acting as a disincentive to invasion (building them along some border so that an overland aggressor would surely be destroyed--essentially, a much more powerful minefield).