The Atlantic Ocean was a major strategic battle zone (the "Battle of the Atlantic") and when Germany declared war on the U.S., the East Coast of the United States offered easy pickings for German U-boats (referred to as the "Second Happy Time"). After a highly successful foray by five Type IX long-range U-boats, the offensive was maximized by the use of short-range Type VII U-boats, with increased fuel stores, replenished from supply U-boats called "Milchkühe" (milk cows). From February to May 1942, 348 ships were sunk, for the loss of two U-boats during April and May. U.S. naval commanders were reluctant to introduce the convoy system that had protected trans-Atlantic shipping and, without coastal blackouts, shipping was silhouetted against the bright lights of American towns and cities such as Atlantic City until a dim-out was ordered in May.