why SACEUR has always been an American officer?
Even since 1950, when the post of SACEUR becomes vacant, the North Atlantic Council asks the President of the United States to nominate an American officer to fill the post. Unlike 1950, however, no specific individual is recommended. Thus while the decision on which nation should fill the post of SACEUR lies with the North Atlantic Council and thus could be changed by the Council, the tradition of having this post filled by a U.S. officer remains strong because:
(1) the United States remains the strongest military power within the Alliance;
(2) having an American officer in charge of the Alliance's military operations symbolises the continuing commitment of the United States to the defence of Europe and reassures those European nations concerned about potential threats to their security;
(3) nuclear weapons remain the ultimate weapon of deterrence for the Alliance, and because the bulk of these weapons come from the United States, it is important to have an American officer in command.
Until recently the second Supreme Commander post in the Alliance, that of the Supreme Allied Commander Transformation (SACT) and previously the Supreme Allied Commander Atlantic (SACLANT), was also held by an American officer, but since 2009 SACT has been a French general.
To balance the leading role played by Americans in the command structure, other key NATO positions have been reserved for non-Americans. Thus the Secretary-General by tradition is always a European and the Chairman of the Military Committee (with the exception of the first one in 1949, when the role of this post was very different) is always either a European or a Canadian.