 Complete the following phrase:  On 19 September 1801 he married the Polish countess Hélène Rostworowska 17831829, with whom he had one son. He became a major general and general aide de camp to Alexander I of Russia in 1802 and was entrusted with diplomatic missions over the following years. Also, in 1805 in Berlin, he negotiated with Austria and Great Britain on behalf of the Prussians to form the three nations into an alliance against France. On 11 November 1805, at 