The U.S. Ninth Army remained under control of 21 Army Group from the end of the operations in the Ardennes until the encirclement of the Ruhr the following April. Until the main Allied crossing of the Rhine in March 1945, the Ninth Army's 137th SRI Company stayed at Valkenberg, Holland. The XII Corps moved to the Ninth Army zone and was joined by the 3258th Signal Service Company (RI) which came from Forges les Eaux near Rouen. It was stationed at Kerkrade on 29 December 1944. On 24 December the XVI Corps' 3257th Signal Service Company put its main station where the 3252d had been at Heerlen, northwest of Aachen. The 137th SRI Company with Headquarters, Ninth Army, and the experienced 3252d Signal Service Company (RI) at Muensterbusch serving the XIX Corps (now commanded by Major General R. S. McLain) were able to help the newly arrived 3258th and 3257th get into production quickly despite the enemy's new cryptic callsign system and the previously unfamiliar enemy units facing the Ninth Army. On 27 January 1945, the 3257th sent a VHF detachment to work with British SIGINT personnel (104 WI Section) at Beek, closer to the front.

In Third Army, the 3256th Signal Service Company (RI) had served temporarily with Millikin's III Corps during the Ardennes battles. When III Corps was transferred to First U.S. Army, the 3259th Signal Service Company (RI) came to III Corps, and took up work at Muhlartshuette; the 3256th went back to Major General Walton Walker's XX Corps in Third Army, in accordance with preinvasion plans.

First Army's 113th SRI Company had been in Limbourg when the Ardennes attack began, and on 18 December 1944 had moved westward ahead of the German offensive. By 18 January 1945 it was back at the old stand in Limbourg. The 3262d Signal Service Company (RI), destined for support of the XIII Corps in U.S. Fifteenth Army, went to the 113th SRI Company for field training in the area west of the Rhine where Fifteenth Army would eventually take over from First Army. The 3262d therefore was available when FUSA crossed the Rhine and built its bridgehead to keep the bridgehead's northern flank under radio surveillance.

Even before the enemy had been forced to abandon his offensive in the Ardennes, he commenced on 1 January 1945 a smaller drive in northern Alsace. When that thrust between the U.S Seventh Army and French First Army had been contained, the Allied Supreme Command insisted that the enemy's large new bridgehead on the western bank of the Rhine – the so-called "Colmar pocket" – had to be eliminated. To accomplish that, General Eisenhower was prepared to strengthen the 6th Army Group by various divisions from a new SHAEF Reserve, and to have both the U.S. Seventh Army and the French First Army committed to the operation.

Persistent reports by agents persuaded 6th Army Group that the enemy kept large German forces either there or across the Rhine available for reinforcement. SHAEF, on the other hand, received contradictory reports from SI. In the end, that SIGINT induced 6th Army Group to commit only one Corps, the XXI (Milburn), from U.S. Seventh Army along with the French. The salient was eliminated expeditiously. The Seventh Army was thus free in January to make better preparations for and to expedite its next operation in the Saar area. That offensive went well.

Initially overcautious, 6th Army Group then became overconfident. Justifiably exulting over its victories in March, it concluded that the German First and Seventh Armies had been decimated and practically eliminated from the war. SI offset the intelligence sources on which such overoptimism was founded. SIGINT indicated that the two German armies would live to fight again. In fact, new German lines of resistance were organized rather quickly in the path of 6th Army Group's advance, which was deterred but not stopped.

After the enemy had been checked and pushed back in the Ardennes and after the smaller attempt in Alsace had also failed, the Allies faced the rigors of a winter more severe than most. The Seventh Army, as the northern command under 6th Army Group, had taken over part of the front previously held by Third Army when that command switched to the Ardennes. In February Third Army did not return to its former zone but remained east of Luxembourg. While it cleared a triangle between the Saar River and the Moselle, captured Trier, and drove the enemy from the Prum River valley and Bitburg, it found the going hard and slow until March.

The German units that faced U.S. Third Army in the Moselle-Saar triangle in March 1945 seemed to be deficient in secure radio communications, for lack of either radio equipment or trained communicators. Frequent intercepts consisted of reports in plain language by units that felt obliged to retreat in order to escape encirclement. The reports often specified the route to be taken in withdrawal. The tactical situation became fluid enough for radio intelligence companies with one of the American armies to collect traffic from German units opposing one of the other U.S. armies along the west bank of the Rhine.

Farther north, the U.S. First Army resumed its advance toward the Cologne plain, while 21 Army Group, with U.S. Ninth Army protecting its right flank, prepared an elaborate operation to cross the Rhine, as the main Allied offensive effort. There the Allies planned to acquire a bridgehead large enough to become the base for a subsequent drive across Germany.

Rather suddenly, the Allied operation farther south gained complete control over the Rhineland and the Palatinate and reached the western bank of the Rhine River. The First Army near Bonn, the Third Army near Coblenz, and the Seventh Army near Worms pierced German defenses and enveloped great numbers of the enemy before they could retire across Rhine bridges that were being preserved for such use. Once the Westwall had been penetrated, the enemy might have withdrawn to the far bank of the Rhine and there deployed for a stalwart defense behind that helpful barrier. Instead, large numbers were caught before they could cross, thus making the defense of the Ruhr and the main areas of Germany that much weaker.

SIGINT kept FUSA aware that the enemy expected to use some bridges for withdrawal and then to demolish them. On the morning of 7 March 1945, elements of Millikin's III Corps were able to cross a tributary of the Rhine, the Ahr River, and to move down its valley to the great, historic stream. Early in the afternoon they found that the Remagen railroad bridge, which had been boarded over for motor vehicles and marching troops, was intact. American troops seized the bridge in a sharp skirmish before it could be blown. By nightfall American tanks and armored infantry had rushed across. During the night, tank destroyers, more infantry, artillery, and antiaircraft units hurried over to take positions in the bridgehead. Thereafter, despite successive counterattacks, observed artillery fire, and repeated air attacks, they held the whole bridgehead, and the bridge itself survived for a week. Supplementary ferries and pontoon bridges were available when the railroad bridge collapsed, and the enemy was eventually pushed back until out of artillery range.

Before the main Allied crossing by 21 Army Group had begun, the First Army's Remagen bridgehead had been extended along the Rhine about forty miles opposite an area also controlled by FUSA. The bridgehead enclosed a section of the autobahn between Frankfurt and Cologne and provided an ample base area for a major attack against the Ruhr.

During the defense and expansion of the Remagen bridgehead, SIGINT identified approaching German units and alerted the Americans to their impending counterattacks. At one point, on 12 March, SIGINT intercepted a communication to the U.S. commander from the German commander near Honnef, where U.S. artillery fire was falling on a monastery. The Germans obtained a suspension of two hours in order to evacuate some 300 children who had been sheltered there.^[150]

As Allied columns pressed northward along the Lohn River toward Marburg, retreating enemy forces gathered to form centers of opposition farther north.

It was still dark one night when word came to one of the forward airfields from Headquarters, Ninth TAC, to send an observer at daylight to check the area near Marburg. The pilot returned from that flight with a report that in the woods there was a huge concentration of German motor vehicles. A squadron of Allied fighter-bombers in the vicinity was redirected to the target; before darkness returned, perhaps as many as 400 tanks, armored cars, and trucks had been demolished.

What had triggered the action? SIGINT! A German message reported the fact that the vehicles were there awaiting fuel for their next move. Decrypted quickly, the word was passed to the chief of staff and A-2, Ninth TAC. An air reconnaissance flight had, as usual, masked the actual source of the intelligence.^[151]

Special intelligence disclosed that the German OB West one day in March 1945 had refused a request from Army Group H to allocate to him all the ammunition being produced by two specified factories in the northern sector of the German defensive front. In declining, he explained that all the medium caliber field howitzer ammunition for the Western Front came from those two producers. Although their existence had been known to Allied intelligence, their importance had gone unrecognized. They had never been struck.

When the SSO, Headquarters, Ninth Air Force called the message to the attention of the director of operations, he decided at once that the factories should be bombed without delay. Within a few hours, they had been destroyed in a bombing attack that also hit two or more similar targets near them.^[152]

The Third and Seventh Armies crossed the Rhine much farther south than the First Army. By 24 March 1945, with the river behind them, Allied forces broke through all defenders and encircled the Ruhr. The Allied high command adjusted to the circumstances by switching the main effort from 21 Army Group to 12th Army Group. The Ninth Army on the north and First Army on the south pressed enemy forces back into the Ruhr and extended their efforts to the east until, at Lippstadt, they were in contact. At that stage, Ninth Army reverted from 21 Army Group's to 12th Army Group's control and completed the reduction of the Ruhr's defenses until a dispirited German force, estimated at about 300,000 – unable either to break out or to be relieved from outside – surrendered.

Meanwhile the several corps of First Army and Third Army under General Bradley' s command and of U.S. Seventh and French First Army under General Devers' 6th Army Group command, suppressed uncoordinated German resistance as they overran much of Germany.

Once they had crossed the Rhine, U.S SIGINT companies began hearing transmissions from German units that were facing the Russians on the Eastern Front. Since such mobile units might shift to face the Allies – in the end some of them did so shift – it seemed important to keep tabs on their whereabouts.

In southern Germany a "Bavarian Freedom Movement" arose as an organized resistance to the Nazi structure which had dominated that region. They got control of a radio broadcasting transmitter by which they could inform the U.S. Third Army, as it approached, of both political and military conditions. They seized control of some towns, which they reported were ready for Allied occupation, and they informed listeners that the citizens and some of the armed forces at Linz were ready to surrender. They alerted the invaders to the fact that military control of the area had been assumed by the German Air Force. They described the removal of roadblocks on Bavarian highways. They even broadcast instructions to the distant German garrisons that had been holding French Atlantic coastal ports to surrender them to the Allies.

To some SIGINT units, conditions in Germany seemed to resemble those encountered in France during the previous summer, except that there were no cheering civilian throngs. Moreover, there were many sharp contrasts between wrecked German towns or disheartening prison camps, on the one hand, and other seemingly untouched areas of Germany that reflected the promise of spring in radiant April weather.

Some U.S. SIGINT units with the armies and corps moved across Germany to Austria or Czechoslovakia. In southwestern Germany, north of Switzerland, the French began to express their nationalism in relations with the Anglo-American Allies. From Italy, radio traffic brought word of the earlier surrenders there and of contacts between Fifth Army and Third Army patrols in Alpine passes.

After the German surrender, SIGINT units were used to monitor communications for evidence of resistance to Allied occupation. Such evidence was lacking. Other army SIGINT units started preparing to participate in the campaigns to effect surrender by Japan. Before that could occur, the Japanese joined the Germans in accepting defeat.

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