Why Britain initially limited submarine targets

Early-war orders generally restricted British submarines to enemy warships, although some boats did not always follow that policy. Surface forces could capture enemy merchant ships and add them to Britain's own resources, an advantage when Britain expected to lose merchant tonnage in another submarine campaign. Submarines could report merchant positions so an available surface ship could intercept them. Restraint also reduced the risk of friendly-fire losses while Britain was gathering allies and establishing recognition signals and procedures. Germany's fleet was comparatively small in 1939 and ranked behind Italy and Japan in balanced fleet capability, making this limited opening policy easier to sustain.