Complex sentence: Coalhouse Fort became an "emergency" battery during the anti-invasion preparations of the early Second World War existing 6-inch guns were replaced in July 1941 with two 5.5-inch guns taken from the ill-fated HMS "Hood" before its sinking.
Split into simpler sentences: Coalhouse Fort became an "emergency" battery during the anti-invasion preparations of the early Second World War. The existing 6-inch guns were replaced in July 1941 with two 5.5-inch guns taken from the ill-fated HMS "Hood" before its sinking.