Yamamoto previously served as the senior vice president of International Programs and Outreach at the National Defense University from 2016 to 2017.

He was appointed by President George W. Bush in November 2006 and presented his credentials to Ethiopian Prime Minister Meles Zenawi in Addis Ababa on December 6, 2006.

[12] From April 22–23, 2006, Yamamoto met with current Chadian President Idriss Déby to discuss Chad's dispute with the World Bank over allocation of its petroleum funds and the possibility of a U.S.-led, United Nations-monitored peace keeping force to end the Chadian-Sudanese conflict.

Yamamoto is the first official in any government outside of Chad to repeat this claim, saying, "It is evident that there was safe haven and logistical support provided to rebel groups.

"[13] Chad produces around 100,000 bpd (barrels of oil per day, 2013 figures) which travels through the Chad-Cameroon pipeline, owned and operated by US companies ExxonMobil and Chevron and Malaysian Petronas.

"[15] Yamamoto met with Ethiopian Prime Minister Meles Zenawi on April 22, 2006, to discuss the ongoing process of democratization in Ethiopia and the Ethio-Eritrea boundary dispute.