In the 1630s, France persuaded Sweden to fight on its side against the Holy Roman Emperor in the vicious, prolonged war between Protestant and Catholic states. The 1631 agreement was the first in a long line of deals between France and Sweden until 1796, which meant 166 years of intermittent support. In some years French subsidies amounted to a startling 20 per cent of the Swedish national budget. But how did the two countries manage to remain allies despite their abiding mutual mistrust?