From the SAIS PhD FAQ:
Can holders of a Bachelors degree apply directly to the Ph.D. program at SAIS?
In order to apply for the Ph.D. program, individuals must hold the SAIS M.A. degree, be a resident SAIS student who has completed at least twelve courses toward the M.A. degree, or be a student or graduate who has comparable academic qualifications from another institution. Holders of Bachelors degrees are encouraged to research the SAIS M.A. degree.
Which advanced degrees are considered to produce graduates with academic qualifications comparable to those of SAIS M.A. graduates? How successful are individuals with law or business degrees in the Ph.D. admissions process?
The Ph.D. Committee accepts applicants who have completed the Master of Arts in International Relations degree at SAIS, as well as applicants who have completed M.A. programs at other institutions. Competitive applicants who have not graduated from SAIS will often hold M.A. degrees in the social sciences, especially in the disciplines of international relations, political science, or history. Such degrees require graduates to have achieved competency in theoretical studies, research methodology, and extensive academic writing.
Advanced professional degrees such as a Juris Doctorate or a Master of Business Administration cannot act as substitutes for advanced work in international affairs. An advanced degree in strategic studies from a military institution would not be considered appropriate for admission to most Ph.D. fields but is sometimes acceptable for admission to the Strategic Studies field at SAIS.