Sperrle had been involved in the war at sea since the first phase of the Battle of Britain. He received an OKL directive on 20 October 1940 ordering him to attack shipping once again in the Thames Estuary. He ordered his dive-bombers into this service, but they were rapidly neutralised in November by a "dynamic defence". The most effective support for the U-boat campaign came from attacking ports, which Directive 23, issued on 6 February 1941, had ordered the German air fleets to do. Direct support to the Kriegsmarine in the Battle of the Atlantic was haphazard; successes were won by accident rather than by design. In late May 1941 "Luftflotte" 3 made a sole contribution to the surface fleet, when it tried, in vain, to save the battleship "Bismarck".