The term "national security" has been in vogue since the beginning of the Cold War. Of course, the concept was referred to in many different ways before that, but only in modern times has it become a watchword. It is convenient to claim issues are either "detrimental to" or "assist in" national security as the circumstance appears to demand. The problem is that everything impacts or is impacted upon by "national security". The current mass of people heading for the U.S. border is said to be a matter of national security. For political reasons this claim of an invasion is countered by indicating they are innocent poor who don't appear to be carrying firearms – so where is the danger to national security? This latter assessment might be called pejoratively a "Trojan Horse" if one was looking for an aggressive definition of the movement of thousands of non-U.S. citizens toward America's southwest border while effectively allowing unwanted elements to imbed themselves among the peaceful migrants.
President Harry Truman was vexed by the circumstance of defining what was to be considered "national security" when he had to face the fact that North Korea's leader, Kim Il Sung, ordered his troops to attack South Korea with the encouragement of Soviet Russia and the newly constituted, Soviet-backed, Chinese Peoples Republic. President Truman believed he had to resist what he saw as Soviet expansionism and also strengthen the United Nations as a peace-keeping force in world affairs. Behind all this, however, was Truman's belief that American national security was being tested by Joseph Stalin who was willing to challenge U.S. post-war dominance in the Pacific.
All this may seem a bit obscure in 2018, but it was real enough in the summer of 1950. If anything, national security - only a bare five years after the end of WW2 - was a more easily understood concept for the American people. They had swiftly become indoctrinated into the characterization of U.S. security now intertwined with international security. This was a serious evolution of American defense consciousness. The concept now had been instilled firmly in the U.S. public's mind. National security had achieved a broad meaning and had become the touchstone of American thought and policy. The Korean War may not have been desired, but it was understood.
For Lyndon Johnson "national security" originally meant staying as uninvolved as possible in Southeast Asia, and especially Vietnam and Laos. During John Kennedy's presidency the U.S. involvement in Vietnam was primarily training Vietnamese forces and a very active covert warfare program. In taking over the White House after the Kennedy assassination, Johnson turned to his friend and advisor, the chairman of the Armed Services Committee, Senator Russell of Georgia. He reminded the new president that as far back as the French defeat at Dien Bien Phu in 1954 they had agreed there was no benefit in the U.S. becoming involved in Saigon's problems. For Johnson this meant that national security should be pursued with minimal military participation in Vietnam. It turned out he had little choice in the matter after President Ngo Dinh Diem was assassinated just before President Kennedy suffered the same fate.
Johnson still felt the best route to preserve national security was to stay away from direct involvement in Vietnam. This proved impossible as the U.S. was obligated to protect Vietnam under the Manila Pact signed by the Southeast Asia Treaty Organization of which the United States was the leading member. That, plus the strong argument of Pentagon military leaders that Vietnam was key to preventing a communist takeover of the region, turned LBJ completely opposite from his original view of what was best for American national security.
There are many other examples where the term "national security" can be manipulated to define both involvement or non-involvement in a dangerous situation depending on the domestic and/or international politics at the time. While there is no imminent threat of war apparent from the burgeoning problem of today's threatened immigrant "invasion", the same argument of protecting "national security" is made. A former Acting Director of ICE has stated that "Border security is national security!" This view appears to be shared by the North American Military Command and the Secretary of Defense.
Clearly there have been instances in the past along the Southwest border of the United States where incursions have brought about military intervention, though not since before World War I. The fact remains that the border is policed by armed federal forces. With the approval of the Mexican Government these federal officers - augmented if necessary, by U.S. military – can pursue and capture anyone seeking to cross the border illegally.
To make the action more palatable it could be a combined Mexican/American force. In any case the issue of "national security" could be claimed by both governments. Again, the matter becomes political rather than one of military action.
There is nothing in the U.S. Constitution nor the Mexican Constitution that says unauthorized masses of people can freely enter and traverse our countries. From a strict security standpoint these citizens of "third countries" are acting, perhaps inadvertently, as a cover and distraction for possible criminal elements. That is clearly a matter of national security for both Mexico and the United States. Recognizing the problem is one thing. Doing something about it is another. Truman and Johnson had their own tilt with issues of national security. At least we are not having to deal with what they did,
The ultimate problem lies in the conflict between the American desire to be regarded as a generous and welcoming country while at the same time enforcing actions to preserve national security and a strict interpretation of the law. It appears such a dichotomy is mutually exclusive.