The loss of the battleships led the US Navy to rely on aircraft carriers as capital ships to make counterattacks against the Japanese, such as the Doolittle Raid in April 1942, and to engage Japanese naval forces at the Battle of the Coral Sea in May 1942 and the Battle of Midway in June 1942. The sinking of the British battleship and her escort, the battlecruiser , further demonstrated the vulnerability of a battleship to air attack, in this case while at sea without air cover. Both ships were on their way to assist in the defense of Singapore when Japanese land-based bombers and fighters found and sank them on 10 December 1941. "Prince of Wales" had the unwanted distinction of being the first modern battleship sunk by aircraft while underway and able to defend herself. Commonly understood as a victory of carriers, Midway showed up deficiencies in Japanese operational planning. Yamamoto, considering his battleships the most valuable units, kept them far to the rear, in line with traditional practice. This placed them too far away to assist Nagumo (and they would have been too slow to keep up with him in any case). Yet, when Nagumo's carriers were sunk, Yamamoto lost an opportunity to salvage something. Carriers, for all their evident potency, were virtually defenseless at night, and Fletcher might have been dealt a crushing blow by "Yamato" the night of 6–7 June, had Yamamoto stayed closer. Initially, when the U.S. entered the war in December 1941, none of the nine Pacific Fleet battleships were available. One was being overhauled at Puget Sound Navy Yard from June 1941 until March 1942. Of the eight at Pearl Harbor, two were total losses and six had been sunk or crippled and were sent to West Coast shipyards for repair and reconstruction. With a top speed of only 21 knots, they would not have been able to keep up with the fleet carriers in any case. The new fast battleships of the and were still undergoing trials. "North Carolina" and "South Dakota" were ready by summer of 1942 and provided anti-aircraft defense during the Eastern Solomons and Santa Cruz Islands carrier battles. By contrast, the Imperial Japanese Navy had the advantage of a dozen operational battleships early in the war, but chose not to deploy them in any significant engagements. The two and two s, despite their extensive modernization and respectable speeds, were relegated to training and home defense, while the two and two were being saved due to fuel limitations for a "decisive battle", which never came.