http://www.nps.gov/calo/U-boats.htm
Visitors to Cape Lookout National Seashore are often surprised to learn that these quiet, undeveloped shores were once an area of death and destruction due to enemy submarines off the coast. In the early days of 1942, U-boats descended upon local waters, wrecking havoc on defenseless ships in full view of the Core Banks. The campaign that followed was marked by instances of ignorance, frustration and confusion that came close to putting the United States out of World War II.
http://www.uboat.net/ops/drumbeat.htm
With the japanese surprise attack on Pearl Harbour on Dec 7, 1941 Hitler was bound by a promise to Japan to also declare war on the US. He did so promptly on Dec 11 and after that all restrictions on German U-boats (which had been attacked and hunted by US convoy escorts in the North Atlantic for the last 5-6 months of 1941 anyway without permission to attack the US escorts) not to attack American shipping were removed. This opened up a whole new field for Dönitz which immediately drew up plans for a devastatingly swift blow on the US eastern seaboard.***
Germany was NOT within its rights to sink anything in British waters, but was sinking merchant ships, a fact not lost on the captain of the Lusitania, who nonetheless forged ahead. The difference was that the Lusitania was not merely a merchant ship, although it carried, supposedly, among other 'goods' bound for England, live shells for the Royal military's use(not a proven fact though as no manifest exists,only ones of conjecture) , but carried both American and British passengers from New York to England.
http://www.pbs.org/lostliners/lusitania.html
http://www.lib.byu.edu/~rdh/wwi/1915/lusitania1.html
http://www.firstworldwar.com/features/lusitania.htm