Source, in full
This military aphorism has been attributed to both von Moltke and Clausewitz, as well as Churchill. It was familiar to President and former Supreme Commander of the Allied Forces in Europe Dwight D. Eisenhower: I tell this story to illustrate the truth of the statement I heard long ago in the Army: Plans are worthless, but planning is everything. There is a very great distinction because when you are planning for an emergency you must start with this one thing: the very definition of 'emergency' is that it is unexpected, therefore it is not going to happen the way you are planning.
Notes from Wikiquote
Speech to the National Defense Executive Reserve Conference in Washington, D.C. (November 14, 1957) ; from Public Papers of the Presidents of the United States, Dwight D. Eisenhower, 1957, National Archives and Records Service, Government Printing Office, p. 818 : ISBN 0160588510, 9780160588518