Even for contemporaries, the conflict that became known in retrospect as the Thirty Years War was a source of bafflement and bewilderment. What began in 1618 as a dispute between rival Catholic and Calvinist claimants to the Kingdom of Bohemia (roughly the modern-day Czech Republic) escalated by fits and starts into a conflict that embroiled most of western Europe. Over the following three decades, war aims metamorphosed. Combatants came and went. Peace treaties proclaimed, prematurely, the end of hostilities – only to be torn up, yet again, as the belligerents returned to the battlefield. By the time the conflict entered its third decade (which is when this new book takes up its narrative), the war had come to involve not only almost all the states of the Holy Roman Empire (the patchwork of principalities, large and small, that sprawled from Dunkirk in the west to the borders of Hungary in the east), but also the three greatest military powers of Europe: France, Spain and Sweden, each of which, at various times, had armies engaged in the fight.

The bigger the war became, the obscurer was the answer to the question of what it was being fought about. Was it a war of religion between Catholic and Protestant? Or a constitutional struggle for states’ rights against the centralising ambitions of the Holy Roman Emperor? Or an old-fashioned dynastic