"This is in response to congressional requests for a memo briefly reviewing how the United States has budgeted for wars in the past. Requesters have asked specifically whether, since World War II, the United States has ever provided funds for wars- or for some other recent military operations- before the operations have started. Based on an examination of previous CRS reviews of funding for wars and for other major military operations, it appears, with one possible exception, that Presidents have not requested and Congress has not provided funding for wars in advance of the start of operations. Rather, Administrations have requested funding after operations have begun, and Congress has subsequently appropriated money to meet specific documented budget requirements."
CRS Memorandum, March 27, 2003