In 1792, he was sent twice, unofficially, to London to avert war, and he was cordially received by Pitt and Grenville. After his first visit, he persuaded the then foreign minister, Charles François Dumouriez, of the importance of having a fully accredited ambassador in London, and the marquis de Chauvelin was duly appointed, with Talleyrand as his deputy. Still, after an initial British declaration of neutrality during the first campaigns of 1792, his mission ultimately failed. In September 1792, he left Paris for England just at the beginning of the September massacres. The National Convention issued a warrant for his arrest in December 1792. In March 1794, with the two countries at the brink of war, he was forced to leave Britain by Pitt's expulsion order. He then went to the neutral country of the United States where he stayed until his return to France in 1796. During his stay, he supported himself by working as a bank agent, involved in commodity trading and real estate speculation. He was a house guest of Aaron Burr of New York and collaborated with Theophile Cazenove in Philadelphia. Burr would later seek similar refuge in Talleyrand's home during his self-imposed European exile (1808–12); however, Talleyrand would refuse to return the favor, Burr having killed Talleyrand's friend Alexander Hamilton in an 1804 duel.