(a) Two-Year Supply.—The Secretary of Defense may not incur any obligation against a stock fund of the Department of Defense for the acquisition of any item of supply if that acquisition is likely to result in an on-hand inventory (excluding war reserves) of that item of supply in excess of two years of operating stocks.

(b) Exceptions.—The head of a procuring activity may authorize the acquisition of an item of supply in excess of the limitation contained in subsection (a) if that activity head determines in writing—

(1) that the acquisition is necessary to achieve an economical order quantity and will not result in an on-hand inventory (excluding war reserves) in excess of three years of operating stocks and that the need for the item is unlikely to decline during the period for which the acquisition is made; or

(2) that the acquisition is necessary for purposes of maintaining the industrial base or for other reasons of national security.