Reading Ron Suskind's "Confidence Men"...: When there was an Under Secretary for Monetary Affairs at Treasury, both the Assistant Secretary for International Affairs and the Assistant Secretary for Domestic Finance reported to this Under Secretary. I believe the Fiscal Assistant Secretary, a position not requiring Senate confirmation, also reported to the Under Secretary, but as a practical matter, the Fiscal Assistant Secretary, who oversees two bureaus, Public Debt and Financial Management Services, has been more an operational position than a policy one.
The Under Secretary for Monetary Affairs position made sense, since it meant that many disagreements, though not all, between Domestic Finance and International Affairs could be resolved without having to appeal to the Secretary.
In the George H.W. Bush Administration, the Under Secretary for Monetary Affairs position was split in two, one Under Secretary for Domestic Finance and one for International Affairs. In the Clinton Administration, the Assistant Secretary for Domestic Finance position was split in two, one for Financial Markets and one for Financial Institutions. Then, I am not sure whether it was in the George W. Bush or the Obama Administration, the Assistant Secretary for International Affairs position was slit in two -- International Finance and International Markets and Development.
The current structure creates more political positions and has devalued the importance of Assistant Secretaries. It can also contribute to more turf fights because of overlapping responsibilities. More important, though, to the functioning of the Treasury Department are the particular people appointed to the various political positions, how well they work together, and the leadership of the Secretary.