It is important not to project the present onto the past. If you want to understand the ontology of the military, you have to look at its ancient origins in their contexts. Our modern military had every specific origins. We have vestiges of the medieval army. The infantry is no longer just "infants" and the "cavalry" became the air cavalry deployed from helicopters. That said, we still have "colonels" (coronel - the crown); and officers are still considered "gentlemen" i.e., a privileged class. But the world we know today was largely made in World War I and its forms were solidified in World War II. That said, the postmodern world of asymmetical warfare will bring some unexpected innovations in technology, but the sociology will be even harder to grasp and manage.
First, while nations maintain the legal and lawful monopoly on force in defense, the US military has a large component of foreigners. It always has. We are a nation of immigrants. The Iraq wars saw the ascendence of Blackwater and similar firms. Nations hire special help when they need it, granted that those contractors do their work under the mantle of lawful authority. In much of Africa, mercenaries are the norm; and central authorities with constitutional powers are weak. That may become a model for the entire planet, if the nation states cannot overpower the non-state ("terrorist") actors. We already have very many gated communities all across America. Think that through to its logical conclusion.
Second, the nature of warfare is changing in front of us right now, today, with the current cyber attacks on Rosneft, Maersk, and others. Make no mistake: this is war. But we (we the people) do not know the real actors, and the actual battle is not being fought with traditional military materiel.
Third, despite easy generalizations, armies are largely unable to halt some kinds of invasions. Think of the "refugees" flooding into Europe. The numbers are like 2% of the population of Germany in Germany, 2% in Sweden, and so on. It would be like America of 1940 having 2.8 million Wehrmacht running around. But, realize, also, that despite barbed wire and guard towers China has a border region with North Korea with thousands of Koreans in Manchuria. (I don't know how many Chinese have escaped into North Korea seeking opportunity...) Most borders are like that. Our lines on the map are all very modern and very arbitrary. The truth may be that traditional armies can only meet traditional threats. The US Army could pulverize an armored column moving north from Mexico. Illegal immigrants? Not so much...
Fourth, the nature of militaries is changing from the perception of theater to a concept of "joint force." If you google the names of your favorite old Army bases, you will see a lot of Joint Force headquarters. Army and Air Force together... Army and Air Force under management of the Navy at an old Naval Air Station, etc., etc. As the leader in training world militaries, America is taking this idea to others. Right now, several state-level National Guards have training agreements with foreign militaries, and have had for over a decade. So, when you say "army" you really mean "military" because the idea of a dedicated land-based force is no longer relevant. It is an easy cliche that the US Army has more ships than the US Navy and more aircraft than the US Air Force. Meanwhile, of course, the Navy has aircraft; but even the Air Force has boats. And they all have robots. The US Marine Corps is actively integrating robots into its assault teams; but again, all services rely on some autonomous vehicles.
Fifth, in response to terrorism and for other reasons, we are going to see the militarization of civil police continue and be extended in depth and range.
That has consequences. While it is an easy claim that both the police and the army "protect the rights of citizens" the fact is that the police have been evolving to perceive their communities as hostiles, as insurgents, as enemy aliens. This will only continue until something comes to change it. That change must be deep. I know that from my decade in private security while completing degrees (AA, BS, MA) in criminology and criminal justice. The so-called "constitutional" role of the police never withstands violation by anyone who thinks they are an authority because they wear a uniform. In private security, I have worked with many cop wannabes. Most of those became private guards because they could not meet the standards of public policing. (Think about that.) But others were former police who retired and now work in private security. Professional though they are, they have not left behind the idea that they are there to enforce the rules and punish people for breaking them. They act like cops; but they are in a business service. It always leads to friction and customer dissatisfaction which then makes actual useful security even harder to achieve. (Perhaps the best cop wannabes are the kids in criminal justice programs waiting to graduate, and get hired by public agencies.)
So, Sixth: when you say "army" you really mean all protective services right down to the school crossing guard.
Think about this: you might claim that you have a Constitutional right to be protected against unlawful searches by the police. And you might be right. But, as a guard working for a private entity where you work or shop, I am not constrained by the Constitution. I can search your vehicle or your briefcase. (My ability to search you personally is limited by most state laws, but there are ways around that.) I do not need a warrant. And, while there are "rules of engagement" and "doctrine" and "commander's guidance" the military does not go around waving papers at people.
(Edited by Michael E. Marotta on 6/28, 3:51am)