Ambassador Johnnie Carson, the head of the State Department's Africa Bureau, formally retired on March 29, ending a 44-year career that included service as Washington's envoy to Nairobi from 1999 to 2003.

Soon after taking office in 2009, President Barack Obama appointed Ambassador Carson as Assistant Secretary of State for African Affairs.

He was the US point-man in Africa during a momentous four-year period that featured the rollback of Al-Shabaab's control over a large swath of Somalia; the passage to independence of South Sudan; undiminished mayhem in eastern Congo; civil conflict in Mali and the Central African Republic; and economic advances and greater democratic stability in several countries, including Kenya.

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Ambassador Carson "did an admirable job", Prof Joel Barkan, a US-based Africa scholar, said on Tuesday. "He put out many fires and made no major mistakes."

Prof Barkan also called Ambassador Carson "the most knowledgeable assistant secretary for Africa in the past 20 years."

Ambassador Donald Yamamoto has been moved up from the number-two post in the Africa Bureau to serve as acting assistant secretary. He has previously been the US ambassador to Eritrea, Ethiopia and Djibouti.

Speculation concerning a potential permanent successor to Ambassador Carson currently centres on Gayle Smith who works in the White House as a special assistant to the president and senior director at the National Security Council.

Ms Smith has an extensive background in Africa, including 20 years of work as a correspondent for news organisations such as the BBC and Reuters.

Also mentioned as a possible candidate is Shannon Smith, the top staff member for Africa at the Senate Foreign Relations Committee. She worked closely with Secretary of State John Kerry when he chaired that powerful committee.

Ambassador Carson, who will turn 70 next Sunday, sought consistently to advance US interests and Obama administration policy in Africa. He was an ardent optimist regarding Africa's potential for achieving prosperity and democracy.

Ambassador Carson also became embroiled in controversies, most recently in response to his warning in February that Kenya would experience unspecified "consequences" in its relations with the US if certain unnamed candidates were elected to lead the country.

The clear reference was to Uhuru Kenyatta and William Ruto, both scheduled to stand trial before the International Criminal Court on charges of crimes against humanity.

Critics charged that Ambassador Carson's remarks represented an infringement of Kenya's sovereignty. The "consequences" warning was also seen by some as a contradiction to comments made a short time earlier by President Obama, who had emphasised Kenyans' right to choose their own leaders and who had made no mention of possible damage to US-Kenya relations.

On Monday, however, the State Department indirectly affirmed Ambassador Carson's suggestion that the election of Mr Kenyatta and Mr Ruto would negatively affect Kenya's relations with the United States.

A department spokeswoman told The EastAfrican that the US "cannot ignore" the ICC indictments and "will calibrate our engagement [with Kenya] accordingly."

In addition to serving as ambassador to Kenya, Mr Carson was US envoy to Uganda (1991–1994) and Zimbabwe (1995–1997). He began his involvement with Africa as a Peace Corps volunteer in Tanzania from 1965 to 1968.

Ambassador Carson's retirement came as no surprise. He had announced in January that he would soon be stepping down.

He was feted in late February at a "Salute to Ambassador Johnnie Carson" organised by the Africa Society in Washington. On that occasion, Congressman Gregory Meeks of New York praised Ambassador Carson as a "history maker."