- See also: Intelligence cycle security
Operations Security (OPSEC) is both a planning discipline, and a set of procedures and techniques, that begins by determining what adversary wants to know about one's plans. The analytic process identifies how the opponent may obtain that information, and how to frustrate the collection of this information. While this is closely related to counterintelligence, it is more specifically focused at protecting specific resources and the manner in which they will be, or are used. It complements, but does not replace, other forms of security.
In the current U.S. definition, it focuses on unclassified sources of information that may be of value to an opponent:
...information generally available to the public as well as certain detectable activities reveal the existence of, and sometimes details about, classified or sensitive information or undertakings. Such indicators may assist those seeking to neutralize or exploit U.S. Government actions in the area of national security. Application of the operations security (OPSEC) process promotes operational effectiveness by helping prevent the inadvertent compromise of sensitive or classified U.S. Government activities, capabilities, or intentions.
During the Vietnam War, a December 1969 capture of a Viet Cong communications intelligence center and documents revealed that they had been getting a huge amount of information using simple technology and smart people, as well as sloppy U.S. communications security.  This specific discovery was by U.S. Army infantry, with interpretation by regular communications officers rather than security specialists; the matter infuriated GEN Creighton Abrams — at the communications specialists. Before and after, there had been a much more highly classified, and only now available in heavily censored form, National Security Agency analysis of how the Communists were getting their information, which has led to a good deal of modern counterintelligence and operations security. 
- Ronald Reagan (1988), National Security Decision Memorandum 298: National Operations Security Program
- Fiedler, David (Spring, 2003), "Project touchdown: how we paid the price for lack of communications security in Vietnam - A costly lesson", Army Communicator
- Center for Cryptologic History, National Security Agency (1993), PURPLE DRAGON: The Origin and Development of the United States OPSEC Program, vol. United States Cryptologic History, Series VI, The NSA Period, Volume 2