The attack on Pearl Harbor and the subsequent German declaration of war on the United States had an immediate effect on the campaign. Dönitz promptly planned to attack shipping off the American East Coast. Dönitz had only 12 boats of the Type IX class that were able to make the long trip to the U.S. East Coast, and half of them had been removed by Hitler’s order to counter British forces in the Mediterranean. One of the remainder was under repair, leaving only five boats to set out for the U.S. on the so-called Operation Drumbeat (Paukenschlag).
The U.S., having no direct experience of modern naval war on its own shores, did not employ shore-side black-outs. The U-boats simply stood off the shore of the eastern seaboard and picked off ships as they were silhouetted against the lights of the cities. The Commander-in-Chief of the United States Fleet, Admiral Ernest King, who disliked the British, initially rejected the Royal Navy’s calls for a coastal blackout or a convoy system. King has been criticized for this decision, but his defenders argue that the United States destroyer fleet was limited (partly because of the sale of 50 old destroyers to Britain earlier in the war), and King claimed that it was far more important that the destroyers protect Allied troop transports than shipping. His ships were also busy convoying Lend-Lease material to Russia, as well as fighting the Japanese in the Pacific. This does not explain the refusal to require coastal black-outs, or to respond to any advice the Royal Navy provided. No troop transports were lost, but merchant ships sailing in U.S. waters were left exposed and suffered greatly. Britain eventually had to build coastal escorts and provide them for free to the U.S. in a ‘reverse Lend Lease’, since King was unwilling (or unable) to make any provision himself.
The first boats started shooting on January 13, 1942, and by the time they left for France on February 6 they had sunk 156,939 tonnes of shipping without loss. The first batch of Type IXs had been replaced by Type VIIs and IXs refuelling at sea from Type XIV Milk Cows tankers and had sunk 397 ships totalling over 2 million tons (as mentioned previously, not a single troop transport was lost). In 1943, the United States launched over 11 million tons of merchant shipping; that number declined in the latter war years, as priorities moved elsewhere.
In May, King (by this time both Commander-in-Chief U. S. Fleet and Chief of Naval Operations) finally scraped together enough ships to institute a convoy system. This quickly led to the loss of seven U-boats. But the U.S. did not have enough ships to cover all the holes, and the U-boats continued to operate freely during the Battle of the Caribbean and throughout the Gulf of Mexico (where they effectively closed several U.S. ports) until July, when the British-loaned escorts began arriving. The institution of an interlocking convoy system on the American coast and in the Caribbean Sea in mid-1942 resulted in an immediate drop in attacks in those areas. Attention shifted back to the Atlantic convoys. For the Allies, the situation was serious but not critical throughout much of 1942.
Operation Drumbeat had one other effect. It was so successful that Dönitz’s policy of economic war was seen even by Hitler to be the only effective use of the U-boat, and he was given complete command to use them as he saw fit. Meanwhile, Dönitz’s commander Raeder was dismissed as a result of a disastrous Battle of the Barents Sea in which two German heavy cruisers were beaten off by half a dozen Royal Navy destroyers. Dönitz was eventually made Grand Admiral of the fleet, and all building priorities turned to the U-boats.