The volume situates an age of intolerance at its core, showing how French persecution and Irish flight reshaped European warfare, fleets of refugees becoming critical contingents in the armies of France and beyond.

The volume starts in an age of intolerance: the Protestant tragedy in France and the Irish tragedy. Tens of thousands of Protestants fled France. Various military and political events of the later 1680s, notably the "Glorious Revolution" of 1688 that chased out King James II and installed William of Orange as King William III of Great Britain led to hostilities in 1689-1691 with Ireland being secured by William. Both had considerable impact on the armies of western Europe. Tens of thousands of officers and soldiers fled France for other nations and also left Ireland for France and later Spain.

Much has been said about the loss of Protestant soldiers to the French army, but less known is that the Irish Catholic military refugees certainly compensated losses in the French army and later also boosted the Spanish army. William's accession to the British throne also meant that Great Britain resolutely now joined the Holy Roman Empire, the Netherlands, Brandenburg-Prussia and other nations opposed to Louis XIV's France that formed the League of Augsburg. In effect, Britain joined Europe in a large military alliance and all-out war broke out in 1689. This was not good news for Louis XIV; although the British army was not formidable, it was an efficient force and a large part of it would, for the first time, join its allies on the continent.