Complex sentence: President Roosevelt ordered the War Department to assert that a bridge in that location, if bombed, would block the East River access to the Brooklyn Navy Yard upstream, a dubious claim for a river already crossed by several bridges, it nevertheless stopped Moses.
Split into simpler sentences: President Roosevelt ordered the War Department to assert that a bridge in that location, if bombed, would block the East River access to the Brooklyn Navy Yard upstream. A dubious claim for a river already crossed by bridges, it nevertheless stopped Moses.