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Operation Drumbeat (January–June 1942)

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. He had only five Type IX boats able to reach US waters for Operation Drumbeat (Paukenschlag), sometimes called by the Germans the "Second happy time."

The US, having no direct experience of modern naval war on its own shores, did not employ a black-out. U-boats simply stood off shore at night and picked out ships silhouetted against city lights. Admiral Ernest King, Commander-in-Chief United States Fleet (Cominch), who disliked the British, initially rejected Royal Navy calls for a coastal black-out or convoy system. King has been criticised for this decision, but his defenders argue 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 it was far more important that destroyers protect Allied troop transports than merchant shipping. His ships were also busy convoying Lend-Lease material to the Soviet Union, as well as fighting the Japanese in the Pacific. King could not require coastal black-outs—the Army had legal authority over all civil defence— and did not follow advice the Royal Navy (or Royal Canadian Navy) provided that even unescorted convoys would be safer than merchants sailing individually. No troop transports were lost, but merchant ships sailing in US waters were left exposed and suffered accordingly. Britain eventually had to build coastal escorts and provide them to the US in a "reverse Lend Lease", since King was unable (or unwilling) to make any provision himself.

The first U-boats reached US waters on January 13, 1942. By the time they withdrew on February 6, they had sunk 156,939 tonnes of shipping without loss. The first batch of Type IXs was followed by more Type IXs and Type VIIs supported by Type XIV "Milk Cow" tankers which provided refuelling at sea. They sank 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 later war years, as priorities moved elsewhere.

In May, King (by this time both Cominch and CNO) finally scraped together enough ships to institute a convoy system. This quickly led to the loss of seven U-boats. The US did not have enough ships to cover all the gaps; the U-boats continued to operate freely during the Battle of the Caribbean and throughout the Gulf of Mexico (where they effectively closed several US ports) until July, when the British-loaned escorts began arriving. These included 24 anti-submarine armed trawlers. 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. As a result of the increased coastal convoy escort system, the U-boats' attention was shifted back to the Atlantic convoys. For the Allies, the situation was serious but not critical throughout much of 1942.