The Nigerian Civil War was a conflict fought between the Nigerian soldiers and Biafran forces lasting from 1967 to 1970. In its course, the US government intervened. The US also rendered relief or humanitarian support to the victims of the conflict, especially those within the Biafran environ or enclave. While this appeared to the US government as the proper step to take toward ensuring global peace and promoting human security, the Federal Military Government (FMG) was against their activities. Available studies on the conflict have detailed the roles played by the US in the conflict, but little is known about the reactions of the parties to this intervention. Combining materials from archives in the US and Canada with data gathered from recently conducted fieldwork in Nigeria, this paper explores how the US arms embargo on Nigeria and Biafra, and the US government's moves against Nigeria's counterproductive blockade on Biafra negatively shaped the relations between Nigeria and US throughout the civil war. More than any other foreign powers which intervened in the conflict, the US policy, due to its flexibility, elicited constant doubts among the Nigerians and Biafrans about the actual position or stance of the great power.