Yugoslavia and the Allies In 1941 when the Axis invaded Yugoslavia , King Peter II formed a Government in exile in London , and in January 1942 the royalist Draža Mihailović became the Minister of War with British backing . But by June or July 1943 , British Prime Minister Winston Churchill had decided to withdraw support from Mihailović and the Chetniks he led , and support the Partisans headed by Josip Broz Tito . The main reason for the change was not the reports by Fitzroy Maclean or William Deakin , or as later alleged the influence of James Klugmann in Special Operations Executive ( SOE ) headquarters in Cairo or even Randolph Churchill , but the evidence of Ultra decrypts from the Government Code and Cipher School in Bletchley Park that Tito ' s Partisans were a " " much more effective and reliable ally in the war against Germany " " . Nor was it due to claims that the Chetniks were collaborating with the enemy , though there was some evidence from decrypts of collaboration with Italian and sometimes German forces . Contact with Yugoslavia . Limited resources meant that in 1942 support for the Chetniks was limited to " words rather than deeds " . The SOE , charged with fostering resistance movements , initially sent Captain D . T . Hudson to contact all resistance groups in September 1941 . Hudson ' s reports on the meetings between Mihailović and Tito ( and their staffs ) were not encouraging , and he sent warnings that the communist Partisans suspected that Mihailović was collaborating with the government of Milan Nedić in Serbia . Contacts with both groups were severed by the first Axis winter offensive , but decrypts of German signals showed that the Chetniks were collaborating with the Italians . This collaboration was based on an old friendship of Serbs and Italians in Dalmatia going back to the times of the Austrian rule . In June 1942 a report by Major General Francis Davidson , Director of Military Intelligence to Churchill , described the Partisans as " extreme elements and brigands " . British Military Intelligence wanted to maintain support for Mihailović at the time that they were watching the progress of the German Operation Weiss against the Partisans , though they started having doubts by March 1943 . Colonel Bateman in the Directorate of Military Operations also recommended supporting the " active and vigorous Partisans " rather than the " dormant and sluggish Chetniks . " An assessment by Major David Talbot Rice of MI3 b in September 1943 confirmed that there had only been isolated anti - German activity by Mihailović and " " the heroes of the hour are undoubtedly the Partisans " " . He recommended that Mihailović should be told to destroy German lines of communication in Serbia , otherwise Tito would be the sole recipient of British aid which they were at long last in a position to deliver . The Signals intelligence had completely changed the view of Talbot Rice and MI3b in six months . When Mihailović was perceived as less effective than the communist Partisans , missions were sent to the Partisans . One of the first of these missions , codenamed " Fungus " , was dropped " blind " in the area of Dreznica and Brinje , north west of Senj on the Croatian Adriatic coast , on the night of April 20 / 21 1943 by a B24 Liberator of No . 148 Squadron RAF , operating from Derna The mission consisted of two Canadian emigrees ( Petar Erdeljac and Pavle Pavlic ) , and Corporal Alexander Simic ( Simitch Stevens ) of the Royal Pioneer Corps . They were found by the partizans and taken to the Croatian Partizan HQ at Sisane Polje , where Erdeljac and Pavlic were recognized by Ivan Rukovina , the Commander of the Croatian HQ , who had fought with them in the International Brigades in Spain . Alexander Simic was interrogated at length by Dr Vladimir Bakaric , the Political Commissaar ( who subsequently became President of Croatia ) before being allowed to establish radio contact with SOE headquarters in Cairo and arrange the subsequent missions of Major William Jones to join Simic at the Croatian HQ and that of Captain Bill Deakin to Tito ' s Headquarters in May 1943 . He was joined the following September by Brigadier Fitzroy Maclean , an SAS officer and also a Conservative Member of Parliament and former diplomat , with good language skills . Maclean subsequently sent a " blockbuster report " to Foreign Secretary Anthony Eden , recommending that Britain should transfer support to Tito and sever links with Mihailović . In 1943 the SOE in England and the Foreign Office wanted to continue support for Mihailović , although as these organisations had only limited access to decrypts they were not so well - informed on the situation there . The SOE headquarters in Cairo ( which was frequently at odds with the London headquarters ) , MI6 , the Directorates of Military Intelligence and Operations , the Chiefs of Staff and ultimately Churchill himself , wanted to switch support to Tito . Churchill ' s sources . Churchill ' s main source was the intelligence decrypts from Bletchley Park which he saw " raw " , as well as intelligence reports and digests . After receiving a signals intelligence digest in July 1943 he wrote that " " it gave a full account of the marvellous resistance by the followers of Tito and the powerful cold - blooded manoeuvres of Mihailović in Serbia . " " Churchill announced his decision to support Tito to Soviet leader Joseph Stalin ( much to his surprise ) at the Tehran Conference in November 1943 , and publicly in an address to Parliament on 22 February 1944 . This address referred to reports from Deakin and Maclean for justification , as the Ultra decryptions from Bletchley Park were secret even after the war Fitzroy Maclean discussed Yugoslavia with Churchill in Cairo after the Tehran Conference . Churchill said that as neither of them intended to live there after the war : " " … the less you and I worry about the form of Government they set up , the better . That is for them to decide . What interests us is , which of them is doing most harm to the Germans . " " Maclean had already reported that " " . . . the Partisans , whether we helped them or not , would be the decisive political factor in Jugoslavia after the war and , secondly that Tito and the other leaders of the movement were openly and avowedly Communist and that the system which they would establish would inevitably be on Soviet lines and , in all probability , strongly oriented towards the Soviet Union . " " However , Maclean had also noticed Tito ' s " independence of mind " and wondered whether Tito might evolve into something more than a Soviet puppet . ) While in England in the spring of 1944 , Maclean discussed Yugoslavia with some of the British officers who had been attached to General Mihailović ' s Headquarters . One of the meetings was at Chequers and was presided over by Churchill himself . " " It was common ground that the Cetniks , though in the main well disposed towards Great Britain , were militarily less effective with the communist Partisans and that some of Mihailović ’ s subordinates had undoubtedly reached accommodation with the enemy . " " And some who knew him best " " while liking and respecting him as a man , had little opinion of Mihailović as a leader " " , though the Cetnik detachments in Serbia at least could be a significant force with " " new and more determined leadership and with better discipline . " " Maclean was also asked to Buckingham Palace to brief King George VI on the Jugoslav situation . He found him as well - informed on the situation as anyone else he had met back in England , and said he " " took an entirely realistic view of it . " " British Intelligence sources . Most of the Signals intelligence obtained by Bletchley Park on the Balkans was initially from Luftwaffe morse code traffic encoded by Enigma ; initially the general Luftwaffe Red key , then various German Army keys . They also decrypted various teleprinter links for high - level traffic : " Fish " ( Vienna - Athens ) then " Codfish " ( Straussberg - Salonika ) , plus medium and low grade hand cyphers . Abwehr , Sicherheitsdienst and railway communications were intercepted and decrypted , providing evidence of resistance activities . For German policy on Yugoslavia , communications to Tokyo from the Japanese Ambassador , General Oshima Hiroshi , were also useful . With the primitive communications infrastructure and the disruption of land communications , the German forces in Yugoslavia relied heavily on radio communications which , unknown to themselves was insecure , so that a 1945 comment was that " " never in the field of Signals intelligence has so much been decrypted about so little . " " While the volume of messages was not great , Bletchley Park also intercepted messages from Tito and from the separate Slovene Communist Party to Georgi Dimitrov , the Secretary - General of the Comintern in Moscow . These messages to Dimitrov continued even after the Comintern was officially dissolved in June 1943 . The volume of Enigma decrypts from the Soviet Fronts and the Balkans declined substantially from the summer of 1944 , but this was more than offset for the Soviet Fronts by success with " Fish " links . Discovery of collaboration . During Operation Weiss against the Partisans in 1943 , the Italian forces used Italian - officered Chetnik units against the communist Partisans despite German objections . Consequently , the German Operation Schwartz against the Chetniks and Partisans was kept secret from the Italians . Pavle Djurisić , one of Mihailović ’ s principal commanders , fell out with Mihailović as he wished to join the Germans against the Partisans , which Mihailović refused to contemplate . Both Axis operations were followed by Bletchley Park in decrypts from the Abwehr ( German military intelligence ) . A decrypted report from General Alexander Löhr , the commander in chief of the German Army Group E in the Balkans , reported on 22 June that 583 German soldiers and 7 , 489 Partisans had been killed , with the probability that the Partisans had lost another 4 , 000 men . Chetnik losses were put at 17 , with nearly 4 , 000 taken prisoner . The contrast between the two resistance movements was stark . However , the decrypts , " " far from providing evidence of Cetnik - German collaboration , continued to leave no doubt that at least at the highest level the Germans remained set on Mihailović ' s destruction . In July Hitler had suggested that the C - in - C South East " [ Löhr ] " should put a higher price on the heads of Mihailović and Tito . " " The most significant report of Chetnik collaboration was the text of a treaty between Lukačević , one of Mihailović ’ s principal commanders , and the German Commander South East in September and October 1943 , In the treaty , which was copied to Churchill , Lukačević agreed to a cessation of hostilities in his area of southern Serbia and joint action against the communist Partisans . British mission . Deakin ' s mission to the Partisans was called " Operation Typical " , and it represented the British General Headquarters in the Middle East . The first parachuted supplies dropped to the Partisans had a very marked propaganda effect despite some bizarre episodes , e . g . a planeload of Atrobin for treating malaria , and a supply of badly needed boots , but all for the left foot . In May 1943 , a signal from Cairo ordered that medical supplies , which had been loaded onto a Handley Page Halifax at Derna were to be left behind , as their despatch would infringe British obligations to the Royal Yugoslav government . The plane ' s crew complied , but loaded all the military items , e . g . , boots , clothing , guns and ammunition , that they could loot at the airfield onto the aircraft . Deakin had urged American representation on the mission , and on 21 August 1943 Captain Melvin O . ( Benny ) Benson of the Office of Strategic Services ( OSS ) arrived , remaining in Yugoslavia for four months . Benson noted in his report " " the giving of credit to the Cetniks for Partisan victories and otherwise referring to them as Patriots , in an attempt to include the Cetniks with the Partisans . " " The crediting of Partisan attacks to Chetniks was also being reported on the BBC . Captain Benson was later replaced by Major Linn ( Slim ) Farish The sending of the Maclean mission on 17 September 1943 placed the relations between Tito and the British on a more formal and senior level . Fitzroy Maclean was the personal representative of the Prime Minister , and his arrival marked implicitly the " de facto " recognition of the Yugoslav National Liberation Army , as the Partisans were formally known . Maclean wondered whether officials in Cairo " " quite realised the difficulties of travel in German - occupied Europe " " , when he was told in an official signal that he was to go to Cairo immediately but that the Partisan delegation could follow later if required . . . ( it turned out that the British delegation was returning from the conference at Teheran via Cairo ) . While away down the coast Maclean was amazed to receive a garbled message from Cairo , with a clear sentence " " King now in Cairo , Will be dropped to you at first opportunity . " " He thought that as part of London ' s gradual rapprochement policy between King Peter of Yugoslavia and the Partisans , the King was to be " " dropped headlong into the seething centre of the Jugoslav cauldron . " " Later he was told that the message referred to their new signals officer , whose surname was King . When the Italians surrendered the mission received a signal from the British General Headquarters in the Mediterranean regarding the Italian forces , which assumed that " " the British mission attached to Tito ’ s headquarters was in some queer fashion in operational command of operational ‘ guerrilla ’ units . " " Similar orders were sent to Colonel Bailey at Mihailović ’ s headquarters and to the commanders of British missions in Greece and Albania , and the episode revealed " " the extent to which our mission had not succeeded in conveying to our superiors the reality of the situation in Partisan - held territory . " " Switching support to the Partisans . The change in Allied support in Yugoslavia from the Chetniks to the Partisans in 1943 was because they were a more effective ally . The public justification at the time was the reports from Maclean and Deakin ; the real source was the signals intelligence decrypts , but they were secret at the time and remained so until the 1970s when the work of Bletchley Park was made public . The change was driven by Churchill and ( British ) Army Intelligence , but was not due to any supposed influence from Randolph Churchill or James Klugman . Churchill ' s son Randolph was on one of the missions to Yugoslavia . Evelyn Waugh accompanied Randolph Churchill , and Waugh put in a report about Tito ' s persecution of the clergy , which was " buried " by Foreign Secretary Anthony Eden . No evidence is given for the suggestion made in the article on Draža Mihailović that Randolph Churchill privately influenced his father to support Tito , and in any case he was recruited by Maclean for his mission after the Teheran Conference , when the decision to support Tito had already been made . James Klugmann was a Communist and was undoubtedly a KGB agent and linked to the Cambridge Five . He joined the Yugoslav section of SOE Cairo in 1942 , where he advocated and lobbied for Tito . But it was stated that " " Whatever lobbying may have been taking place in Cairo , it would have been the overwhelming evidence of the Bletchley Park decrypts , Churchill ' s favoured source of intelligence , which persuaded Britain ' s wartime leader that Tito and his Partisans were a much more effective , and reliable , ally in the war against Germany . " " Captain Bill Deakin , who led the first military mission in 1943 and was caught up in the Battle of the Sutjeska ( hence the title of his book ) had been Churchill ’ s researcher and librarian in the thirties . Allied bombings . The United States Army Air Force ( USAAF ) and Royal Air Force ( RAF ) bombed many Yugoslav cities and towns during the Axis occupation . These attacks included intensive air support for Partisan operations in May – June 1944 , and a bombing campaign against transport infrastructure in September 1944 as the " Wehrmacht " retreated from the Balkans . This latter operation was known as Operation Ratweek .