Sophisticated eavesdropping - not radar, as Hitler's high command suspected - was the secret weapon that broke the German U-boats' strangle-hold on the Atlantic sea lanes in World War II, according to formerly top secret documents just turned over to the National Archives.

Allied codebreakers intercepting and decoding German radio messages eventually sifted out those between Adm. Karl Doenitz's command and its U-boats, pinpointing their locations at sea and sealing their doom.

The Germans, according to the unnamed National Security Agency writer of a formerly top secret report on the Battle of the Atlantic, "apparently never really believed" that the Allies could be intercepting and decoding such sensitive messages..

"Knowing that the U-boat had lost its secret strength and had become the hunted instead of the hunter," the report says in discussing the desperate countermeasures the Germans tried to develop in 1943, "the U-boat command persistted in reducing the mystery of Allied locations to radar" or to some other technical advance that could be countered if only German technicians would supply the right gear. Astonishingly, the possibility that the German high command's coded messages were being intercepted and read was not given serious consideration.

Doenitz apparently thundered at technical experts all over Nazi Germany, demanding that they come up with the new equipment needed to combat the Allied radar he believed was responsible for locating and sinking his U-boats. Help is coming, he kept radioing his U-boat commanders, exhorting them to "Attack!" to win the war for Germany.

The admiral was not overstating the value of cutting the sea lanes between the Allied arsenal in the United States and the fighting units in Europe, Africa and the Soviet Union. Said the National Security Agency on this point:

"The rapid depletiion of the Allied merchant fleets at the source of Allied supplies constituted the greatest single threat of defeat ever encountered in our war with Germany."

But Doenitz's insistence on directing every step of the Battle of the Atlantic through radio commands gave the edge to the Allies even before they had the ships and planes needed to move against the U-boats.

"Heading points and operational plans were ordered by radio after the U-boats had put to sea," noted the U.S. government's hush-hush eavesdropping outfit, the National Security Agency.

U-boats were not only told where to go after they had put to sea but also when they were expected to arrive in their heading area," the NSA said in its formerly top secret report which, despite the gray government prose, conveys a sense of incredulity about this way of directing the U-boats, whose survival depended on stealth.

Armed with the intercept information netted by their eavesdropping effort, code-named Ultra, Allied commanders were able to divert their conveoys away from wolfpacks of U-boats in the Atlantic, starting in January 1943.

"For the greater part of January," notes the NSA in reporting on this turning of the tide of the Battle in the Atlantic. "U-boats groups swept and reswept, made course and reversed course and found nothing" because the eavesdroppers had been able to warn the merchant ships where the U-boats were.

By the spring of 1943, the NSA report said, the Allies had mobilized enough ships and planes to attack the German submarines rather than just evade them. U-boat losses rose to a rate of 1 1/2 boats a day in 1943, alarming Doenitz and his command and instilling a sense of dread in the U-boat crews at sea. Now they were being surprised by the Allies rather than the other way around.

The NSA report quotes the intercepted coded message from Doenitz to his submarine skippers in May 1943 in which he had to admit that the Battle of the Atlantic was being lost but that new weapons to regain the initiative were on the way:

"By use of his radar, the enemy has now once more gained a few lengths on us in his effort to deprive the U-boat of its most important attribute, its invisibility.

"I am fully cognizant of what this has meant for you in your difficult battle with enemy escort and defense. Be assured that with all my strength as commander in chief I have undertaken and shall undertake every means at all possible to alter this situation as soon as I can.

"Experimental stations in and out of the navy are working to improve your arms and instrument equipment. I expect of you that you will continue your determined fight against the enemy, and that against his wiles and technical innovations you will pitch your ingenuity, your ability and your obdurate will to dispose of him no matter what he does.

"In the Mediterranean and in the Atlantic, commanders have proved that the enemy even today has weaknesses at every turn and that his auxiliary devices are in many instances by no means so effective as they at first appear to be, proved that one is determined to achieve something in spite of them.

"I believe that I shall soon be able to give you better weapons for this hard battle of yours."

In another intercepted and decoded message, Doenitz's demand for desperation tactics by his U-boat skippers comes through with chilling clarity. Don't worry about the Allied planes, said Doenitz of bombers which were blowing U-boats out of the water by the dozen in 1943, but just use your anti-aircraft guns (which were virtual pop guns). "Then the plane will soon stop attacking," Doenitz assured his skippers.

However, both the submarine skippers and the German high command had to concede the Battle of the Atlantic was lost - at least for the time - and in 1943 concentrated on other sea lanes not as crucial to the Allied war effort.

Very late in the war, the NSA/notes, German technicians came up with ways to restore the invisibility of the U-boat; among them, the snorkel breathing device that enabled a submarine to remain under water for long periods. But the improvements came too late.

On May 5, 1945, the defeated Doenitz radioed to his skippers this message which, like the other ones that had proved so fatal to the German navy, was intercepted by the Allies' Ultra listening net:

"Six years of U-boat warfare lie behind us. You have fought like lions. An overwhelming superiority in materiel has forced us into a very narrow space. From this small basis a continuation of our battle is no longer possible . . . U-boat men, unbroken and unshamed, you are laying down your arms . . . keep your U-boat spirit, with which you have fought bravely . . . long live Germany."

The NSA said that of the 489 German U-boats sunk by Allied forces at sea starting in January 1943, the U.S. Navy sank 63 "with the direct aid" of Ultra radio intercepts plus "some 30 more with the indirect aid of Ultra." The Ultra information, as noted earlier, also saved untold tons of Allied war supplies as convoys were routed around German U-boat wolfpacks.

The NSA, in a formerly top secret volume entitled "Allied Communication Intelligence and the Battle of the Atlantic," noted that despite the success in driving the German U-boat out of the Atlantic, the fleet as a weapon was never really destroyed.

"It should be borne in mind that the U-boat arm was not in May 1945 defeated at sea," concludes the NSA report.

"The pre-snorkel U-boat had been decisively swept from the Atlantic in the summer of 1943, but the loss was made good and the U-boat reappeared in force. The power of this fact in the imagination and memory of a possible future German navy will not be easy to estimate."