'Polish notices are everywhere.

Polish shops have sprung up; and Polish waitresses serve in the cafes and restaurants -" This sounds familiar. There can't be many towns in Scotland where the supermarket doesn't have a substantial corner of world foods devoted to unfamiliar pickles and potted meats with too many consonants on the label and too much fat in the jar.

But this isn't Scotland in 2012. This is Tehran in 1942, and part of the astonishing, tragic, uplifting, but surprisingly little-known story of the Polish nation in the Second World War. The surprise is inverse to a strongly persistent mythology. Where there is a deep need to suppress an ethnic narrative, we tend to replace history with romance. Genuine affection is mixed with guilt. Hence the idea, still sustained in history textbooks, that Polish cavalry – foolish! brave! doomed! – charged at German tanks in 1939, a brief and unrepeated incident turned into a national military characteristic.

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Britain went to war over Poland in 1939, and then did surprisingly little to help her, preparing instead for war in the West. It was as if Austen Chamberlain's 1925 statement that "no British Government will or ever can risk the bones of a British Grenadier" for Danzig had been proven wrong, and then suddenly right again.

The Allies, and America, had been committed to the creation of a free Poland in 1918, but seemed inconsistent in practice 20 years later. Poland became the only country under continuous occupation from the first day of the war until the last. She was (again) parcelled out by the encircling powers, a Fourth Partition enforced by two cynically pragmatic partners who would two years later treat her as a battlefield. Even at the end of the war, the Soviet Union declined to restore Poland to her independence, and the West seemed unwilling or unable to commit wholeheartedly to it.

Poland's wartime sufferings were more reminiscent of the Thirty Years War than of the modern age. Her population was massacred and scattered, to Britain, Persia, Palestine, South America, everywhere except the US and Australia, it seems. She became the proving ground of the Holocaust, housing the most fearsome of the extermination camps: Auschwitz, Belzec, Chelmno, Sobibor, Treblinka. Some 90% of Poland's Jews were liquidated. Those who survived included the troublingly but understandably militant first generation of Israeli politicians.

And while this was going on, there gathered round the Poles a certain fatal glamour. Girls graduating from smart schools in the south were warned off gin and Polish airmen. The exiled fliers' image as feckless charmers – this before oversexed American aces were "over here" – was at odds with their actual skill as combatants.

The Poles had enjoyed the first, sharp taste of truly modern war and had much to teach the British and the French. They were by no means a shivering diaspora but an active fighting force which made a significant contribution in Norway, the Middle East, at sea and in the air. That there was no commensurate recognition of their ongoing national status was both inevitable and worrying. Poland was also unique in the war in having both a government in exile and an underground government at home. There was no Vichy solution to the running of the occupied country.

Halik Kochanski's extraordinary achievement is to bring together the threads of a story only known in fragments or through well-meaning fictional versions like Ian Serraillier's The Silver Sword. This is the first fully comprehensive account in English of Poland's war. It is also a brilliant exercise in historiography, showing how the myths and misconceptions that surround the Polish story were constructed and reinforced, how the Polish-born British historian Lewis Namier (né Ludwik Niemirowski) had shaped Lloyd George's attitude to Poland and done so in the most negative of lights.

Namier can easily be cast as a disaffected exile, a hater of his own origins, but Kochanski reveals that some of his strongest beliefs about his homeland, and not least his refusal to believe that Poland could be a Great Power in the East, were based on fact and reality. Far from being a strong and integral power in 1939, brutally raped back and front by Nazis and Soviets, Poland was a country with little infrastructure, with deeply problematic ethnic minorities all claiming attention, without much political integrity (in the sense of wholeness, rather than principle), a romantic abstraction enshrined in Woodrow Wilson's 14 Points but not really (yet) a consistent polity. In creating the "Polish corridor" to the sea, internationalising Danzig and splitting off large numbers of ethnic Germans, "Versailles" painted a target on her back.

There has always been an aura of betrayal and cover-up around Poland's recent history. The Katyn massacre of Polish officers by the NKVD was covered up when the Soviet Union became an ally. There was even a suggestion – most infamously made in Rolf Hochhuth's 1967 play Soldiers, which Kochanski surprisingly does not mention, perhaps because of Hochhuth's association with David Irving – that Churchill ordered the assassination of wartime Polish leader Władysław Sikorski, who died in a plane crash in 1943.

Whether it was a plot or simply ill-luck, it helped contribute to a strangely distorted sense of Polish wartime history. Kochanski neither debunks nor sensationalises. She has no ideological axe to grind, and makes balanced use of family experience and interview material as against the official record and a handed-down sentimental consensus. The truth is far more powerful than the legend. It's great history writing.

The Eagle Unbowed: Poland And The Poles In The Second World War

Halik Kochanski

Allen Lane/Penguin Press, £30