Books: Ne Plus Ultra

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THE ULTRA SECRET by F.W. WINTERBOTHAM 199 pages. Harper & Row. $8.95.

What if? is history's forever teasing and unanswerable question. What if Marshal Ney's troops had not sat eating lunch before Waterloo while the Duke of Wellington retreated to safety? Etcetera. To all such historic posers must now be added questions raised by a retired British group captain named Frederick Winterbotham. What if a Pole working in a German factory had not defected to the Allies in 1938, bringing with him the first construction details of the Nazis' coding machine, called Enigma? And what if British cryptographers had not eventually cracked Enigma's supposedly unbreakable coding system, which throughout World War II was to carry all the German high command's secret wireless traffic? Would Britain have fallen? Would the Allies have lost the war?

After 35 years of officially imposed silence, Winterbotham reveals in The Ultra Secret that British intelligence did crack the code. From 1939 onward Churchill and later Roosevelt, Eisenhower and other Allied leaders were virtually reading over Hitler's shoulder. The whole system of deciphering Enigma's signals and relaying the intelligence was called the Ultra Operation. It sometimes produced translated copies of Hitler's orders to his generals within an hour of their original transmission. Little wonder that Churchill once called Ultra "my most secret weapon."

Terrible Decision. This is not a book on code breaking. Winterbotham passes over the mechanics of Enigma to deal with the Allied use of its output. Dissemination of Ultra intelligence had to be limited to only a few senior commanders, lest the Germans (and later the Japanese, who also used Enigma machines) should discover the awful truth.

No Ultra intercepts could be retransmitted. Allied field commanders had to mask privileged Ultra knowledge with conventional intelligence gathering (like air reconnaissance) to keep the enemy from suspecting odd coincidences. In 1943, when American pilots, armed with precise information from Ultra, shot down the Japanese navy's Pacific chief, Admiral Yamamoto, over the island of Bougainville, London protested to Washington about the lack of an adequate cover plan. Fortunately the Japanese were too shocked to notice.

The overriding military necessity of preserving Ultra's secrecy sometimes led to Allied tragedy. On Nov. 14, 1940, for instance, Ultra picked up German bombing orders for a blitz of the cathedral city of Coventry, well before the attack was due. Winterbotham relayed word to Churchill, who then faced a "terrible decision": whether or not to evacuate Coventry and almost certainly give Ultra's secret to the Germans. Churchill's choice doomed a city.