Era beginning/end
Though early evidence of W.I.L.L.O.W.I.S.P. lurking in the shadows before this point can be found throughout history, this date is attributed to the organizaiton's initial creation. Such was most likely prompted by the implementation of the Office of Strategic Services by President Harry S. Truman when he signed the National Security Act of 1947 three years prior. Making their presence known (only slightly) to the agency that would soon come to be referred to as the CIA, W.I.L.L.O.W.I.S.P., they assisted (and, sometimes, worked against) them.