With Hitler's invasion on 1 September 1939, Poland's two decades of independence came to an abrupt halt. Over six million Poles lost their lives during the Second World War – almost twenty per cent of the population. Quite apart from the terror of everyday life, with its street seizures and summary executions, the ghettoisation and persecution of the Jewish population and the ruthless murder and deprivation carried out in the concentration camps, any expression of Polishness could prove fatal. Polish institutions and organisations were liquidated by the occupying Nazi forces. An early signal of German intent was a summons to leading academics of the Jagiellonian University in Kraków to attend a meeting on 6 November 1939: the 183 professors who turned up were immediately imprisoned and sent to the Sachsenhausen concentration camp.

The Germans were determined to eradicate the Polish nation, although they can hardly have been surprised that, as already shown by nineteenth-century insurrections against occupying powers, Poles should now demonstrate equally incredible and sometimes foolhardy bravery in defending their nationhood. A Polish resistance movement, known as the Home Army (AK – ‘Armia Krajowa’), was immediately mobilised and Poles created for themselves ‘a State within a State’. As Adam Zamoyski relates: