In early June 1944 at Litakovo in western Bulgaria, Major Frank Thompson, a British officer captured in uniform, was executed by royal Bulgarian soldiers. He was only 23 years old. About six weeks earlier Thompson had parachuted into southern Serbia, then under Bulgarian occupation. He had subsequently accompanied a force of Bulgarian communist partisans in their ill-fated attempt to set up a liberated partisan area in south-central Bulgaria. In late May 1944, the ragged column was surrounded by royal Bulgarian forces and overwhelmed. Thompson's partisan companions were executed along with him. His wireless operator, Sergeant Kenneth Scott, was spared and later freed.

At first, the Bulgarian communist resistance celebrated Major Thompson as a hero; a railway station north of the capital Sofia was named in his honour. But then strange rumours began to circulate. Some said that he had been an imperialist agent. Others claimed that he had been an unwitting stooge, whose regular reports to Special Operations at Cairo enabled his taskmasters to betray the column to the Bulgarian government. In Britain, Thompson was sometimes ridiculed, sotto voce, as a communist fantasist, who had treasonably or vaingloriously disobeyed strict instructions not to cross the frontier into old Bulgaria. To some, he became a tragicomic figure, a kind of "Lawrence of Bulgaria", who planned to ride into Sofia at the head of ragged band of workers and peasants. To others, the communist Thompson was the archetypical establishment traitor who had infiltrated SOE and got his just deserts. Neither of these two interpretations was entirely corrected by Stowers Johnson's book of the episode, Agents Extraordinary, which was published more than 20 years ago.

Frank Thompson was the elder brother of E. P. Thompson, best known for his campaign against nuclear weapons and for his radical studies of pre-industrial and industrialising Britain: books such as The Making of the English Working Class and Whigs and Hunters have achieved classic status. Thompson's characteristic method was to eschew establishment Whig pieties, and rigid Marxist teleologies for a self-consciously subversive perspective which rescued the losers and small people from the "enormous condescension of posterity". It is not hard to see the same approach at work here: Edward Thompson is trying to rescue his elder brother from the enormous condescension of London clubland and doctrinaire Stalinists. In this he has succeeded admirably.

The picture that emerges of Frank Thompson is sympathetic, but also credible. The first-born of a radical missionary couple, he grew up in a household where such figures as Gilbert Murray, St John Philby, and Robert Graves were regular vistors. At Winchester College he became a keen linguist and classicist, acquiring skills that were to stand him in good stead during the war. His communist sympathies were powerfully influenced by the agony of republican Spain during the civil war; and he was an ardent supporter of the anti-appeaser Jack Lindsay during the famous Oxford byelection of 1938. During the war itself, Thompson proved himself a courageous soldier who demanded no sacrifice of others that he himself was not prepared to make.

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None of this makes Beyond the Frontier simply an exercise in fraternal piety. On the contrary: it is vintage Thompson. As in Whigs and Hunters, a comparatively minor incident is used as a vehicle to make broader points about oral history, establishment cover-ups, partisan warfare, and even the beginning of the cold war. By means of a subtle blend of witness testimony and the official files available to him, Thompson attempts to peel back the various layers of distortion to get closer to the truth of what really happened to his brother.

Far from being an unauthorised adventure, Thompson shows the mission to have been part of SOE's original strategy to destabilise Axis-held southeastern Europe. Just before Thompson crossed the border, however, British policy towards royalist Bulgaria changed: rather than toppling the regime, it now aimed to absorb it "as a going concern" - to quote a contemporary phrase - into the Allied cause. Thompson's mission, therefore, had become redundant before it had even started, but could not be aborted because his radio was defective; this also disposes of the various communist myths that he had either treasonably or unwittingly betrayed the column by wireless. After his capture Thompson became an embarrassment to the British government who were not - so the argument runs - heartbroken to be rid of a communist officer and potential troublemaker. According to this reading, Major Frank Thompson was not so much a victim of the fight against fascism as an early casualty of the cold war.

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Persuasive as this interpretation is for the period up to Thompson's capture, it does not satisfactorily explain his subsequent execution. For how are we to account for the survival of the wireless operator? The difference in treatment cannot be explained just by Sergeant Scott's potential usefulness to his captors, which was not greater than Thompson's. Surely the answer lies not so much in any British sins of commission or omission, but in the attitude of the royalist Bulgarians. And they had good reason to hate and fear Major Thompson.

Edward Thompson makes no secret about the fact that his brother was a communist. He defines the basis of this commitment as "an internationalist antifascist contestation". This included support for Europe's beleaguered Jews, opposition to appeasement, and support for the republican side during the Spanish civil war. All this was admirable enough. But tucked away at the end of this definition is a reference to "identification I in a more cloudy and self-deceiving way, with a Utopian construct known as the Soviet Union".

Aye, there's the rub. Thompson assures us that none of his brother's actions "followed an 'instruction' from King Street (Communist party headquarters) or from the Comintern". Even so, there is little evidence that his fundamental sympathy with Soviet Russia was lastingly dented either by Stalinist mass murder within Russia, or by the Nazi-Soviet pact of 1939. So when Major Thompson volunteered himself as a liaison officer to the Yugoslav and Bulgarian partisans - as many noncommunist British officers did also - he was not just making a contribution to the defeat of Hitler's Europe. He could also reasonably be inferred to have thrown in his lot with murderous ideologues, whose socialist utopias were to be achieved through massacre and repression.

Edward Thompson takes a dim view of royal Bulgaria, which by 1943 was "a place somewhat like El Salvador". "I need not tell you", he remarks, "that Bulgaria in the 1930s and early 1940s was not a pleasant western democracy". Doubtless, it was not. But then, the communist partisans were not proposing to create a pleasant western democracy either: they planned a dictatorship of the proletariat. This was a project which promised - and proved to be - every bit as inhumane as that of their local Balkan adversaries. Unsurprisingly, the communists were opposed by a wide spectrum of landowners, bourgeoisie and peasants, not all of whom were necessarily in sympathy with the royal government.

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When Fitzroy Maclean - probably the best-known British liaison officer in wartime Yugoslavia - asked Churchill about his indifference to Tito's domestic policy, the latter famously asked whether Maclean was planning to live in Yugoslavia after the war. In Maclean's case, the answer was no: his romantic sympathy for the partisans coexisted with a fundamentally Tory outlook. Frank Thompson's case was different: he was not merely a British officer creating opportunistic mayhem for the Germans; he was a true believer whose total identification with the partisans is evident from his letters in the field right down to his defiant performance in the dock.

Let us try to see things from the other point of view. The Bulgarian government saw the second world war not in terms of any global "contestation", but, like virtually every other grouping in the region, as an opportunity to settle old scores. Indeed, the royal government managed to save almost the entire Jewish population in Bulgaria proper, a feat similar to that of the Danes, but executed in much more difficult circumstances. Of course, the royal government welcomed the dismemberment of Yugoslavia in 1941 as a chance to recover Macedonia, which it occupied until 1944. It had no particular quarrel with Great Britain, with which it was at war, save that it was historically perceived to be a supporter of Serb, Greek and Rumanian territorial claims at her expense. Ironically, Bulgaria was not, technically, at war with the Soviet Union. But like all central and southeastern ancien regimes, it had a powerful fear of communism.

In a revealing exchange cited by Stowers Johnson, the Bulgarian court asked Frank Thompson: "By what right do you, an Englishman, march into our state and make war against us, not only as a soldier but politically". Not only as a soldier but politically - this is a key phrase to understanding the fate of Edward Thompson's brother. The royal Bulgarians executed this captured officer in uniform because they saw him not primarily as an enemy soldier - like Sergeant Scott - but as an ideological warrior and an agent of international communism, akin perhaps to a Soviet commissar. After all, Thompson was a fluent Bulgarian and Russian speaker with a British radio set; his doubtless genuine bafflement about the Russian set discovered with the column did not convince his captors. In any case, Thompson made no secret of his communist views during the interrogation and trial. Finally, the column not only saw but also conducted themselves as class strugglers: they killed not only enemy soldiers but also symbols of authority such as tax collectors; and they took innocent civilians hostage or pressed them into service as guides. Edward Thompson calls these and similar practices "desperate marginal actions in a civil war (what many statesmen today would undoubtedly describe as 'terrorism')". Who will rescue Bulgarian tax collectors from the enormous condescension of history?

Another aspect to Thompson's death is hardly mentioned in this book: the Macedonian dimension, which is crucial to understanding partisan and royal Bulgarian policy. For Thompson's expedition was on the northeastern periphery of a many-sided struggle whose roots famously stretched back into the 19th century. To royalist Bulgarians, indeed to all nationalist Bulgarians, including many Bulgarian communists, the Macedonians were ethnic kin to be rescued from Serbian and Greek subjection. They were therefore deeply suspicious of Macedonian autonomists among the Yugoslav partisans - such as their leader "Apostolski" - who sought their salvation in a federal Yugoslavia under Tito. They were even more suspicious of Titoist schemes to bring the whole of Macedonia - including prewar Bulgarian and Greek territory - within the Yugoslav umbrella. It is therefore hardly surprising that the royal Bulgarians regarded the partisan column which started from Apostolski's bases in Bulgarian-occupied southern Serbia/Macedonia with such fear.

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Frank Thompson did not die in vain. Beyond the Frontier proves that his foray into Bulgaria was no amateurish folly but a calculated blow against the enemy for which this soldier bravely - and uncomplainingly - paid with his life. But as a communist, too, Frank Thompson had achieved much. In the short term, the raid had provoked the Germans and the royal Bulgarians into atrocious reprisals against a previously largely complaisant population; this helped to prepare the ground for future partisan activity. In the long term, the exploits of Thompson's column helped the Bulgarian communist party to claim a rather inflated partisan pedigree after the war. Not many British officers deserved to have a communist-bloc railway station named after them.

Brendan Simms is a fellow of Peterhouse, Cambridge.

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