Arturo Valenzuela's return to academia, which had been announced in May, leaves the US without much needed diplomatic leadership in Latin America and the Caribbean.
Trade opportunities and strengthened ties top the agenda as President Obama flies to Brazil, Chile, and El Salvador over the next five days.
Secretary of State Hillary Clinton will push for new sanctions on Iran's nuclear program in Brazil as part of a five-country trip to Latin America.
As the Rio Summit came to a close Tuesday night, Latin America leaders announced that they would start a new regional bloc that excludes the US and Canada.
An accord that would allow the Honduran Congress to vote to allow Manuel Zelaya back into the presidency is 'dead,' the ousted president said. US Sen. Jim DeMint has said the US will recognize Nov. 29 presidential elections regardless of whether Zelaya is returned to office, which many nations around the world have demanded.
The military ouster of Honduras's president, Manuel Zelaya, has led to a deep divide between Democrats and Republicans.