How a fifty-metre sea-level change would remake maritime geography

How would a fifty-metre rise or fall in sea level during the twentieth century change naval and global strategy?

It would remake coastlines, ports, river systems, invasion routes, and industrial regions so thoroughly that almost every major power would need a different maritime strategy.

A fifty-metre fall would leave the English Channel shallow and expose banks and shelves, while changing the ports and estuaries on which fleets and trade depend. A rise of the same scale would flood low-lying states and coastal industrial areas and remove some Pacific islands from the twentieth-century campaign map. In either direction, dockyards and slipways would lose their relationship to the shoreline, and existing river routes and harbour sites could cease to work. A proposed Russian advantage rests on having more inland space to adapt, but freshwater contamination and the location of industrial and agricultural areas complicate that expectation. Japan's mountainous terrain would make disruption to its infrastructure especially difficult. These regional estimates do not establish a precise map of the resulting world.