Words matter. Different words are usually used to convey different meanings. I learned recently that "international" was coined only in the early 19th century by Jeremy Bentham. There are two terms that I tend to use interchangeably, though I realize that technically they are supposed to be different.
"Foreign policy" is the action of a state beyond its borders, usually with other states. Those actions encompass the full spectrum from trade and culture to war. "National security" deals with the various policies a state pursues to protect and support its people and institutions, especially in relation to other nations and peoples.
Those are crude definitions, but the terms also reflect longstanding debates and policy preferences among scholars and policymakers. "Foreign policy" is an older term, long used as a study in history that concentrated on diplomacy and wars. "Military history" is a subfield dealing with armed forces and their use in conflicts. In my own field of political science, "foreign policy" has long been split between students of international relations in general -- especially the causes of war and conflict resolution -- and those concerned with the policies of particular countries. Academic specialization makes it hard to bridge those fields. The same is true in government: foreign policy in America is seen as the purview of the President and State Department. National security tends to be equated with "defense" and largely assigned to the Pentagon, with White House oversight and ultimate control.
An interesting historical development occurred in the United States in the 1930s. Civilian scholars such as Edward Mead Earle at Princeton began urging more academic and governmental attention to the foreign policy aspects of military policy. Earle complained about the "water-tight compartments" in both institutions and urged more attention to "effective coordination of military with foreign and domestic policies." Implicit in his argument was the notion that war was too important to be left to generals and that civilian control of the military required more knowledge of military matters by civilian officials. The term they adopted for this new field of study was "national security."
Franklin Roosevelt, who in 1919 had proposed a Joint Plan-Making Body from the State, War and Navy departments, to define wartime objectives and force levels, as president established an Army-Navy planning board in the White House. Wartime experience led civilian and military experts to call for greater coordination of the armed forces and between the military and the diplomats. They embraced the umbrella term of national security.
Accordingly, when Congress finally enacted a comprehensive law in 1947 putting the armed forces under a single secretary of defense and creating a White House level advisory body and establishing the Central Intelligence Agency, they called their law The National Security Act of 1947 and their panel the National Security Council. Moreover, that law specifically said that the function of the council was to advise the President on "the integration of domestic, foreign and military policies relating to the national security."
Implicit in that new terminology was a policy preference for coordination across once-separate institutions. That's why "national security" has largely supplanted "foreign policy" when describing America's international relations. What still hasn't been widely recognized, however, is that the field properly includes foreign economic policy as well as defense and traditional diplomacy. Since I teach in what historically has been called the "American foreign policy" department, I feel obliged to use that narrow term even when I mean the broader one.But I recognize the difference and wish the U.S. government took a more comprehensive approach in its policymaking.