 He opposed the Mexican–American War as a war of aggression to further slavery.He declined re-election in 1851, and, in March 1851, was appointed by President Millard Fillmore, Minister to Brazil and also accredited to Uruguay, Argentine Confederation, and Paraguay.He was directed by the Government to visit Buenos Aires, Montevideo, and Asunción, and make treaties with the republics around the Río de la Plata and its tributaries.