They seemed unstoppable. Bad news just kept coming. In the first few months of the New Year Japan defeated US forces in the Philippines and beat the British in Hong Kong, Singapore, and Rangoon, the capital of Burma.

Many of the place names were unfamiliar even to seamen such as Chuck and Eddie. To the American public they sounded like distant planets in a science-fiction yarn: Guam, Wake, Bataan. But everyone knew the meaning of retreat, submit and surrender.

Chuck felt bewildered. Could Japan really beat America? He could hardly believe it.

By May, the Japanese had what they wanted: an empire that gave them rubber, tin, and – most important of all – oil. Information leaking out indicated that they were ruling their empire with a brutality that would have made Stalin blush.

But there was a fly in their ointment, and it was the US Navy. The thought made Chuck proud. The Japanese had hoped to destroy Pearl Harbor completely, and gain control of the Pacific Ocean; but they had failed. American aircraft carriers and heavy cruisers were still afloat. Intelligence suggested the Japanese commanders were infuriated that the Americans refused to lie down and die. After their losses at Pearl Harbor the Americans were outnumbered and outgunned, but they did not flee and hide. Instead they launched hit-and-run raids on Japanese ships, doing minor damage but boosting American morale and giving the Japanese the unshakable feeling that they had not yet won. Then, on 25 April, planes launched from a carrier bombed the centre of Tokyo, inflicting a terrible wound on the pride of the Japanese military. The celebrations in Hawaii were ecstatic. Chuck and Eddie got drunk that night.

But there was a showdown coming. Every man Chuck spoke to in the Old Administration Building said the Japanese would launch a major attack early in the summer to tempt American ships to come out in force for a final battle. The Japanese hoped the superior strength of their navy would be decisive, and the American Pacific Fleet would be wiped out. The only way the Americans could win was to be better prepared and have better intelligence, to move faster and be smarter.

During those months, Station HYPO worked day and night to crack JN-25b, the new code of the Imperial Japanese Navy. By May they had made progress.

The US Navy had wireless intercept stations all around the Pacific Rim, from Seattle to Australia. There, men known as the On The Roof Gang sat with headsets and radio receivers listening to Japanese radio traffic. They scanned the airwaves and wrote what they heard on message pads.

The signals were in Morse Code, but the dots and dashes of naval signals translated into five-digit number groups, each representing a letter, word or phrase in a code book. The apparently random numbers were relayed by secure cable to teleprinters in the basement of the Old Administration Building. Then the difficult part began: cracking the code.

They always started with small things. The last word of any signal was often OWARI, meaning end. The cryptanalyst would look for other appearances of that number group in the same signal, and write ‘END?’ above any he found.