The secret messages of Marshall Tito and General Mihailović

The Kingdom of Yugoslavia was one of the states that were created when the old Austro-Hungarian empire collapsed at the end of WWI. The country covered a large area in the Balkans but was politically unstable since it was made up of a diverse group of peoples (Serbs, Croats, Slovenes, Montenegrins).

Yugoslavia was part of the Little Entente organized by France. Although its foreign policy was pro-Allied it did not declare war on Germany in 1939. The defeat of France in 1940 caught the Yugoslav leaders by surprise and forced them to adopt a pro Axis policy. This change however was opposed by a group of military officers and in March 1941 a coup replaced the regent Prince Paul with General Dušan Simović. This maneuver (thought to be organized by the British) infuriated Hitler and he ordered that the country was to be destroyed as a political entity. In April Yugoslav troops were quickly overrun by German forces and a period of occupation and internal strife began.

During the occupation the old antagonisms between ethnicities (Serbs vs Croats) and political movements (Right vs Left) resurfaced and led to a multisided civil war. The Chetniks of General Mihailović fought the Communist Partisans of Marshall Tito and both attacked the collaborationist government of Milan Nedic, the German and Italian occupation troops and the Croat forces of Ante Pavelić.

All sides took to heart the motto ‘the enemy of my enemy is my friend’. This meant that at times some resistance group would make a deal with the occupation authorities and agree to leave them alone so both could attack another group. The results of this widespread conflict were mass destruction of property and loss of lives as each group attacked the villages that supported their enemies.

During the period 1941-44 the Germans mounted major operations against the resistance movements but they could not destroy them. In their war against the Chetniks and the Partisans however they took advantage of signals intelligence. The resistance groups used codes that could not withstand a serious cryptanalytic attack and their cipher clerks made many mistakes that facilitated solution. By reading the traffic of Tito and Mihailović the Germans could build up the OOB of their organizations, identify important personalities and anticipate enemy operations.

At the same time the British also used cryptanalysis in order to monitor the internal Yugoslav situation and decide which resistance group they should give supplies to. Their ability to decode the Enigma cipher machine meant that they could use German military messages to see if the information coming from the Chetniks and the Partisans was corroborated by official German reports. They also read Chetnik and Partisan messages including the clandestine traffic between Moscow and Tito (this program was called ISCOT).

German effort

According to post war interrogations of Army personnel and the archives of the Army’s signal intelligence agency Partisan and Chetnik communications were intercepted and decoded both by forward units in the Balkans and by the central department in Berlin.

KONA 4 (Kommandeur der Nachrichtenaufklärung - Signals Intelligence Regiment) had fixed and mobile units intercepting and decoding traffic from the Balkans and the Middle East. Specifically British, French, Turkish, Bulgarian and resistance movements communications. Yugoslav communications were also worked on by small detachment under Lieutenant Wollny (Nachrichten Aufklärung Zug ‘W’) based in Belgrade.

Report of KONA 4 for 2nd quarter 1944:

The few systems that resisted attack were handled by the Balkan department (Referat 6) of OKH/Inspectorate 7/VI headed by councilor Bailovic. In the period 1943-44 the department was also able to solve the codes used by Mihailović for his communications with the Yugoslav government in Exile and some the traffic of British liaison officers in Yugoslavia and Greece.

Mihailović traffic

Most of the Mihailović traffic was single and double columnar transposition with the same key being used for both cages. The keyword was taken from a novel. This traffic was solved thanks to the stereotyped beginnings and endings of the reports (many messages ended with the signature ‘GENERAL DRAZA MIHAJLOVIC’) or operator mistakes and it was possible, in some cases, to retrieve the book that was used as a key source. According to Army cryptanalyst Herzfeld (assigned to the central department at that time) 70% of the messages were solved. The success rates given by Reudelsdorff (a member of the Wollny unit) were 95% for simple transposition and 55% for double transposition.

The reports of KONA 4 show that double transposition became the main system in late 1942 and solution required the analysis of a large volume of traffic.

In 1943-44 lots of traffic was solved but this was accomplished thanks to the combined efforts of KONA 4 and Referat 6 since the cipher security of the Mihailović movement underwent a significant improvement. The statistics for KONA 4 are the following:

1st quarter ’44: 4.500 messages.

2nd quarter ’44: 4.400 messages.

3rd quarter ’44: 2.500 messages

4th quarter ’44: 776 messages and 99 advance reports issued (S-Meldungen/Sofortmeldungen).

The traffic allowed the Germans to create a card index of Chetnik personalities and follow the movement of their units in Yugoslavia. Some of the messages were particularly noteworthy since they showed that Mihailović greatly distrusted his British liaison officers and suspected them of colluding with Tito.


Special traffic Mihailovic-Yugoslav government in Exile

After Yugoslavia was occupied by Axis forces, in April 1941, King Peter II and many politicians and government ministers were able to escape abroad and they constituted the Yugoslav government in Exile. Their official representative in the country and head of the resistance movement was General Draža Mihailović.

Mihailović and the Yugoslav government in Exile kept in contact through the use of enciphered radio-messages. These communications were investigated by the German codebreakers and in May 1943 they were able to solve the first messages of the special traffic (sonderverkehr) between Mihailović and the London based government. The cryptosystem used was double transposition and the solution was difficult and required time consuming work by the personnel of Referat 6.

Report of Referat 6 - May 1943:

The special traffic to London (and later Cairo) continued to be solved till January 1945. The monthly average for the period 1943-44 was about 50 messages with the maximum achieved in June ’44 at 113 messages.

Tito traffic

The traffic of Tito’s units was enciphered with a numerical monoalphabetic cipher consisting of a one or two digit number substituted for each letter. A short repeating additive sometimes based on a key-word was used for superencipherment. These systems offered limited security and could not secure the large volume of traffic from Partisan units. The reports of KONA 4 show that in 1943 this traffic was easily solved and valuable information was gained on the organization and operations of the Tito movement.

In 1944 Partisans traffic continued to be read successfully till May ’44 when the Partisans started using non-repeating additive sequences for enciphering their code (this system was called ‘Novo Sifra’). For a while the new system proved secure but regional commands obviously found it cumbersome and reverted to old insecure systems. Thus the Germans could read a lot of Partisan traffic even after mid ’44.

The statistics for KONA 4 are the following:

1st quarter ’44: 7.600 messages.

2nd quarter ’44: 14.000 messages.

3rd quarter ’44: 12.500 messages.

4th quarter ’44: 10.024 messages.

The most important messages were those from Tito’s HQ to the regional commands in Croatia, Montenegro, East Bosnia, Dalmatia and Slavonia. The intercepted messages allowed the Germans to identify the Partisan personalities, the OOB of their units and anticipate enemy operations. They also showed that Tito had an extensive espionage network throughout the country.

Traffic Tito-Moscow

Marshall Tito had close relations with the Soviet Union and during the war he was in constant contact with Moscow through a radio link. This traffic obviously became a target for the Germans and they investigated it in 1944 without finding a theoretical solution.  

Report of March 1944:

Some messages however were read thanks to captured material. In May ’44 the Germans tried to capture Tito during operation ‘Knight’s Move’(Operation Rösselsprung). Although they did not succeed in their primary objective they were able to gather cipher material from his abandoned headquarters. This material was used to solve some of the Moscow traffic in June ’44:

It should be noted that other German agencies might have also investigated the foreign traffic of the Tito movement. For example the codebreaking department of the German Foreign Ministry – Pers Z solved Comintern traffic during the war.

Apart from cryptanalysis the Germans had other ways to gain information. Messages between Tito and Moscow were sent by radio and by courier. During the war some of these couriers fell into German hands with the result that Tito’s political maneuvers could be followed. In 1943 they were surprised to learn that Moscow had ordered him to assist the German forces in case of Allied invasion of the Adriatic coast!


British liaison officers

The British authorities kept in contact with Tito and Mihailović through liaison officers sent by the intelligence services SIS and SOE. These small teams transmitted traffic by radio to their controlling stations in Cairo, Egypt and Bari, Italy. The cryptosystems used were double transposition and the War Office Cypher, enciphered with one time pads.

Some of the encoded radio traffic of British officers in the Balkans was exploited by the Germans. They were able to read messages both through captured material and by cryptanalysis. The reports of KONA 4 show that some cipher material was captured in the field and messages read. For example in 1943:

In 1944 the investigation of the cipher material recovered from Tito’s HQ showed that the British officers had a copy of the War Office Cypher, used with one time pads, plus double transposition keys.

 

Field units had to rely on captured material in order to read British agents transmission but this was not necessarily true of the central department. The reports of Inspectorate 7/VI show that this traffic (special traffic to Cairo with indicator GESH) was first solved in June by Referats 6 and 12:

 

Traffic continued to be read till November 1944 but it seems this was mostly from the team assigned to the headquarters of General Mihailović and from the liaison officers in Greece.

September ’43:

April ’44:

July ’44:

 

Italian effort

The Italian Army’s Intelligence agency SIM (Servizio Informazioni Militari) had a cryptanalytic department that successfully solved Yugoslav codes from the 1930’s up to 1941. During the occupation of Yugoslavia, the Slavic desk turned its attention to the communications of partisan groups and by mid 1943 had solved two systems used by the Chetniks and one used by Tito’s Partisans.

British effort

The Brits first established contact with resistance movements in September 1941. During the period 1941-44 they monitored the internal situation in Yugoslavia by decoding German military, police and Abwehr messages. Messages between Tito and Moscow were read from 1943 onwards. From the autumn of ’43 Internal traffic of the Chetniks and the Partisans was monitored and decoded from a station in Bari, Italy.

The Enigma traffic in 1942 revealed that Mihailović had become a problem for the German occupation authorities and there were plans to capture him. Intercepted messages showed that the Chetniks were fighting against the Germans and had captured the city of Foca (a town south-east of Sarajevo). This news contradicted the version given by European papers (probably with Moscow’s blessing) that attributed the victory to Tito’s partisans.

By 1943 the British were concerned by Mihailović’s decision to conserve his forces for the period of liberation. The SOE organization was particularly critical of the Chetniks and favored the Partisans. In the first half of the year Enigma messages provided detailed coverage of the major anti-partisan operations Weiss and Schwarz. All these operations were inconclusive as they inflicted heavy losses on the resistance movements but failed to destroy them or capture Mihailović and Tito. The Enigma traffic revealed that the fighting ability of the Chetnik units was inferior to that of the Partisans and that there was cooperation between Chetniks and Italians. However no indication of Chetnik collaboration with the Germans was found. Another benefit of the decoded Enigma messages was the discovery that the Germans were reading Chetnik and Partisan traffic.

In the course of the year Enigma messages showed that German authorities considered the Chetniks as a threat and wanted to arrest Mihailović however at the same time there was evidence of cooperation between Chetnik groups and the Germans against the Partisans. By the end of 1943 these reports led to a change in British policy as Mihailović was considered to be holding back his units while the Partisans were engaged in major operations. The Partisans had also won battles against the Chetniks in the ongoing civil war. The British thus increased supplies to the Partisans while Mihailović received virtually no supplies after November 1943. In September 1943 the Italian surrender meant that Italian divisions laid down their arms and many were disarmed by the Partisans. This was a huge boost of their combat power as overnight they captured heavy weapons and ammunition. Combined with the political decision to back Tito this meant that the Partisans were now the rising force in Yugoslavia.

In 1944 decoded messages showed that Mihailović was hard pressed by the Partisans and the loss of British support. As his forces failed to hold down Axis units the Allied military missions attached to his forces were recalled by May ’44. At the same time the Partisans increased their sabotage operations and attacked garrisons of satellite troops. Major German operations in Zagreb, Sarajevo area and Bosnia were again inconclusive as they inflicted heavy casualties but did not destroy the Partisans or permanently remove them from these areas. In the second half of ’44 the country fell into Tito’s hands as the Germans evacuated Southeastern Europe.

Conclusion

The Axis occupation of Yugoslavia unleashed the dormant nationalistic forces inside the country and led to a civil war between different ethnic and political groups such as the Croats, Serbs, Communists and Royalists.

The Germans and Italians tried to destroy the Chetniks and the Partisans through military force but they could not concentrate large enough forces to cover the whole country. In their campaign against the resistance movements they had to rely on signals intelligence. According to postwar interrogations the interception and exploitation of Chetnik and Partisan communications produced good results.

From their side the British also used signals intelligence to guide their policy versus Tito and Mihailović. Initially they followed the Yugoslav government in Exile in supporting Mihailović. However the information from German traffic showed that Tito’s forces were defeating the Chetniks in the ongoing civil war and that they were not holding back their forces but were attacking Axis units and destroying rail lines.  Since the British goal was to divert as many German units as possible to Yugoslavia it makes sense that they chose to back Tito in 1943.

This decision has been criticized postwar and several authors claim that communist ‘fellow travelers’ or Soviet agents had a hand in the change of policy. Although this might be true up to a point there is no doubt that British policy was guided by the signals intelligence coming from German and Yugoslav sources. The decoded messages showed that the Partisans were able to fight and survive against German and Italian offensives and at the same time defeat the Chetniks. In 1943-44 with operation ‘Overlord’ coming up the Allies needed to draw as many German units as possible away from Western Europe. In the eyes of the British leadership the Partisan movement in Yugoslavia could achieve this goal. Of course this meant that postwar the country would fall to the communists but it seems that was a price that Churchill was willing to pay.

Sources: TICOM reports I-51, I-115, I-205, CSDIC/CMF/Y36 , CSDIC (U.K.) SIR 1704, ‘British intelligence in the Second world war’ vol3 part1, ‘European Axis Signal Intelligence in World War II’ volumes 1,2 and 4, ‘The Secret Front: Nazi Political Espionage 1938-1945’, ‘The history of Hut 6’ vol1, 'Action This Day: From the Breaking of the Enigma Code to the Birth of the Modern Computer’, Kriegstagebuch Inspectorate 7/VI, reports of KONA 4 for 1943-44.

Acknowledgements: I have to thank Ralph Erskine for information on the British exploitation of Partisan and Chetnik codes and Randy Rezabek for the reports of KONA 4.

Additional information: For more information on the use of signals intelligence by the Germans check the articles ‘The Key to the Balkans: The Battle for Serbia 1944’ and ‘The German ‘ultra’: signals intelligence in Yugoslavia 1943–1944’ by Gaj Trifković.