At the start of the eighteenth century Louis XIV needed to remit huge sums of money abroad to support his armies during the War of the Spanish Succession. For this essential task the French government turned to the services of a number of leading international bankers, men who could provide foreign exchange on a vast scale through a multitude of European financial centres. However, the demand for foreign exchange was unprecedented and the remittance system was utterly opaque, with government ministers ignorant of its workings and almost impotent before its power. As a consequence, the bankers’ highly secretive activities proved ruinously expensive to the French state. This book explains how the bankers moved French money across much of Europe for Louis XIV, how the foreign exchange system was overloaded, and why the demands of war led in 1709 to a massive banking crash the French state could not prevent.