From "Rockets and People, Volume 1" by Boris Chertok (and Chertok
was there all the time):
What were the Allied and Soviet intelligence services doing during these years? One can forgive our intelligence service for their ignorance of the scope of operations in Germany on missile armaments.The service had been crushed twice— first under Yezhov, and then again under Beriya.
It seems improbable,but nevertheless, before May 1943 neither reports provided by agents, nor information from prisoners of war, nor air reconnaissance, nor any other type of intelligence gathering had provided reliable information about the true scale of operations to develop the new secret weapon.
As a result of the Peenemünde bombing in August 1943, the Wehrmacht decided to create a backup research test range in Poland to continue the optimization of the A-4 and to bring it to the point of reliability for combat. At the same time, the military was tasked with intensifying the training of troop formations to service combat launchers. To accomplish this, Himmler proposed using the SS Heidelager test range in Poland, which was located in the Debica area between the Vistula, Wisloka, and San Rivers.
Thanks to the actions of the Polish partisans and underground, the British intelligence service had received valuable information concerning the test range in Poland. They had even managed to send an airplane to pick up missile parts gathered by the partisans from missile crash sites. The Brits had also obtained the remains of rockets that crashed in Sweden.
There was no time to waste, and Churchill appealed for help directly to Stalin.
FROM CORRESPONDENCE BETWEEN CHURCHILL AND STALIN
Personal Top Secret Message from Mr. Churchill to Marshal Stalin:
1. There is reliable information to the effect that for a substantial period of time the Germans have been conducting missile tests from an experimental station in Debica, Poland. According to our information this projectile has an explosive charge weighing approximately 12,000 pounds, and the effectiveness of our countermeasures depends to a significant degree on how much we can find out about this weapon before it can be used against us. Debica is located on the route of your victoriously advancing troops and it is completely possible that you will seize this site in the next few weeks.
2. Although the Germans almost certainly will destroy or haul off as much of the equipment located at Debica as possible,you will probably be able to obtain a great deal of information when this area is in Russian hands. In particular, we hope to find out how the missile is launched because this will enable us to determine the missile launch sites.
3. Therefore, I would be most grateful, Marshal Stalin, if you could give the appropriate instructions concerning the preservation of the equipment and facilities in Debica, which your troops might capture after seizing this area, and if you would then provide us with the opportunity for our specialists to study this experimental station.
13 July 1944.
Churchill and Stalin exchanged six telegrams in 1944 on the participation of British specialists in an expedition to the German test station in Debica.
Stalin gave instructions to allow the Brits to inspect the test range, though not as quickly as Churchill would have liked. Due to the particular secrecy of the correspondence between Churchill and Stalin, the texts of the letters were not accessible until long after the deaths of both leaders.
In many respects for our future activities, Churchill’s appeals to Stalin were truly decisive.
If not for his letters, our victorious army would have moved right past these Polish marshlands and forests without investigating what the Germans had been doing there. With the help of the Brits, we were able to recover A-4 missile parts for the first time.