When the succession of Cleves and of Julich, so long expected and already discounted by the treaty of Halle (1610), was opened up in Germany, the great war was largely due to an access of senile passion for the charms of the princesse de Cond.
On his arrival at Madrid he found the princesse des Ursins all but omnipotent with the king, and for a time he judged it expedient to use her influence in carrying out his plans.
4to); Lettres a sine Princesse d'Allemagne sur quelques sujets de physique et de philosophie (St Petersburg, 1768-1772, in 3 vols.
Another task to which he set himself immediately after his return to St Petersburg was the preparation of his Lettres a une princesse d'Allemagne sur quelques sujets de physique et de philosophie (3 vols., 1768-1772).
His first book was the fictitious Memoires d'Anne de Gonzague, princesse palatine (1786), thought by many people at the time to be genuine.