External bulges were added to improve both buoyancy to counteract weight increase and provide underwater protection against mines and torpedoes. The Japanese rebuilt all of their battleships, plus their battlecruisers, with distinctive "pagoda" structures, though the received a more modern bridge tower that would influence the new . Bulges were fitted, including steel tube arrays to improve both underwater and vertical protection along the waterline. The U.S. experimented with cage masts and later tripod masts, though after the Japanese attack on Pearl Harbor some of the most severely damaged ships (such as and ) were rebuilt with tower masts, for an appearance similar to their contemporaries. Radar, which was effective beyond visual range and effective in complete darkness or adverse weather, was introduced to supplement optical fire control. Even when war threatened again in the late 1930s, battleship construction did not regain the level of importance it had held in the years before World War I. The "building holiday" imposed by the naval treaties meant the capacity of dockyards worldwide had shrunk, and the strategic position had changed. In Germany, the ambitious Plan Z for naval rearmament was abandoned in favor of a strategy of submarine warfare supplemented by the use of battlecruisers and commerce raiding (in particular by s). In Britain, the most pressing need was for air defenses and convoy escorts to safeguard the civilian population from bombing or starvation, and re-armament construction plans consisted of five ships of the . It was in the Mediterranean that navies remained most committed to battleship warfare. France intended to build six battleships of the and es, and the Italians four ships. Neither navy built significant aircraft carriers. The U.S. preferred to spend limited funds on aircraft carriers until the . Japan, also prioritising aircraft carriers, nevertheless began work on three mammoth "Yamato"s (although the third, , was later completed as a carrier) and a planned fourth was cancelled. At the outbreak of the Spanish Civil War, the Spanish navy included only two small dreadnought battleships, and . "España" (originally named "Alfonso XIII"), by then in reserve at the northwestern naval base of El Ferrol, fell into Nationalist hands in July 1936. The crew aboard "Jaime I" remained loyal to the Republic, killed their officers, who apparently supported Franco's attempted coup, and joined the Republican Navy. Thus each side had one battleship; however, the Republican Navy generally lacked experienced officers. The Spanish battleships mainly restricted themselves to mutual blockades, convoy escort duties, and shore bombardment, rarely in direct fighting against other surface units.