FORT BELVOIR, Va. – Rushing into the room, an infantry soldier sees a dog to his right his attention quickly turns to two men firing from behind a waist-high

In an American civil war would the US Army operate in the conflict or would it be only the National Guard?

EDIT: I misread the question, it was not specifically about THE (actual, historical) Civil War, but A (hypothetical) civil war in the US. Let me add these thoughts to what I already wrote below, most of which is still useful as reference to this question:

  1. OF course in any internal conflict within the US, the US Army would have a role. In fact, they have the PRIMARY role under various laws, regulations, and operations planning. This only makes sense, since the Army is the primary land force of the US Armed Forces, and is also the biggest of the Armed Forces.
  2. In a hypothetical future US civil war or domestic disturbance, all US military assets would be involved, from the Coast Guard to the Air Force and everything in between.
  3. Under the Federalist system, our Republic allows significant leeway for subordinate political subdivisions, i.e., States.
  4. The States maintain their own Army and Air National Guard (see below) organizations, of varying sizes and capabilities. For example, California’s National Guard has something like 37,000 members. That’s about the size of many Nations’ entire military and police forces, and is also about the size of the Royal Air Force of the UK…
  5. In the political, social, and economic unrest that leads up to a civil war, there will be increasingly frantic, and powerful, efforts by local, State, and Federal authorities to contain the unrest and try to return to a peaceful state.
  6. Once a locality determines it cannot maintain control or provide safety for its citizens, it calls upon the next higher political level, so a city typically calls for a county, which calls to the State, which would call upon the Federal Govt. All efforts to police, and provide emergency services, starts at the local level and continue until the local authorities cry uncle to the next level up. If the entire State is burning, or otherwise the State believes it cannot control the situation, it will ask the Federal Govt for assistance.
  7. This is the genesis of the Posse Comitatus Act (PCA). PCA prevents the Federal Govt from just sending in troops any time it pleases. Even when the situation is out of control, the requests for assistance are supposed to come from the State, not just random Federal military movements…(there are exceptions, but let’s stick with the main theme for now…)
  8. Once the Federal Govt begins to move assets, whether police, emergency services, wildfire fighting assets, Public Health teams, morgue teams, or the military, everything works within the National Response Framework (see: National Response Framework (FEMA)), in conjunction with certain State and Federal operational plans prepared in advance by the Department of Homeland Security, DoD, and Northern Command (the part of the DoD that commands all US military assets used within the US itself).
  9. By the time a State has asked for Federal intervention, most States will have mobilized their own National Guard, and reallocated as much of their Statewide police, fire, and EMS services to cover down on as much as possible of the unrest and problems. At this point, the National Guard will be under “State Active Duty (SAD),” and the State will be paying for it all out of pocket (although perhaps will be reimbursed for some of it later…out of the scope of this answer).
  10. At any time, if the President, at the advice of his Secretary of Defense and other advisors, may, especially with the consent of the Governor(s) involved in the unrest, “Federalize” the National Guard (whether all or part of each State’s), calling the militia into Federal service.
  11. AT which time, for all intents and purposes, the Federalized Army National Guardsmen and their units have become part of the US Army (technically, The Army of the United States), and the Federalized Air Force Guardsmen will become part of the The Air Force of the United States, and will be commanded as necessary by Big Army (and Big Air Force, respectively), and Northern Command (over all Federal forces within the continental US). Their State will lose all direct control over them…but could ask or try to guide decisions to their favor, which is not unreasonable, but the US Army and Northern Command will then decide if what the State is asking for can be done, or if the National Guard forces are needed elsewhere, to do something else, perhaps all the way across the Country.
    1. As a side note: at least 2 states have functional Naval Militias, which are authorized under US law as the maritime equivalent of the Army and Air Force National Guard (as the land and air militia). Several states also have “State Defense Force” versions of their Naval Militia, meaning it is not Federally recognized, but it still has certain capabilities available only to the State. The Federally-recognized Naval Militias are also available for recall to active duty in both a State or Federal capacity.
  12. At this point, the President usually has called upon the “Insurrection Act,” a very old law that allows the President to protect Federal interests such as military bases, postal offices, Federal court houses, and the civil rights and livelihoods of all US citizens. So the Insurrection Act is invoked, orders to disperse are given, and if not obeyed within the time limits given, that allows the Federal Govt to mobilize whatever assets (military or other) are necessary to quell the situation. An example was the LA riots: the Insurrection Act was invoked after California asked the President for help, and US Marines and Soldiers were ordered into the affected areas to restore order and protect the civil rights of all US citizens there, along with a huge task force of Federal/state/local police and other first responders, and elements of the California National Guard (some Federalized, some not, I believe, because National Guardsmen who are not in a Federal status are not bound by PCA, and can perform any law enforcement or other duties as tasked, under State law). As soon as the riots died down, and California said it had reestablished control over the areas, the President ordered the Federal troops out and extinguished the Insurrection Act.
  13. But the Act could have remained in place…say for a more widespread civil unrest, that spread and became more and more violent. The President could then extend the Act everywhere in the US, if required, allowing the US military to operate anywhere required…and the PCA is no longer in play. AT that point, the US Govt is effectively cutting all local governments out of the equation, and taking direct control of affected areas, and Federal troops can be used to perform all tasks required, from combat to delivering the mail to providing medical services, to arresting and detaining law breakers…even to holding military tribunals. But we know how that worked out in Guantanamo, so in reality, I doubt any military tribunals would be set up except in a very carefully controlled environment. Better to reestablish local courts and law enforcement, and hand over law breakers for them to deal with, or at worst, use Federal law and Federal courts to handle them in the civilian court system.
  14. What about martial law? Well, that is effectively the situation when the civilian courts cannot function…when that happens, some type of control and court system must be in place, and to ensure that control is reestablished and civil authority can resume normal court and law and order operations, the US military might have to seize control of the chaos. Martial law in the US is a HUGE political hot potato…nobody even likes to use the words, let alone envision the circumstances where it is required or used. Stopping Martial law was largely what the PCA was designed to do..but the PCA cannot cover all contingencies, and when it is no longer in effect, such as during Homeland Defense operations and in areas covered by the Insurrection Act, there may be martial law in place, where the military provides law and order, or there may still be civilian courts and law and order but not to the point where they fully protect the rights of all citizens. Nobody in the US military even likes to talk about martial law, as it is so antithetical to what we stand for, it’s like talking about disobeying our Oaths and making a military coup to take over…nasty business, indeed.
  15. At the point when civilian authorities have lost control over an area, and the US military (including Federalized National Guard units) moves in, the situation changes from “Defense Support of Civilian Authorities,” or DSCA, where the US military is supporting civilian requirements, and operates generally under their guidance (although not command!), to a very different situation called “Homeland Defense,” where the DoD becomes the supported Agency, and the entirety of the national assets are directly or indirectly supporting DoD operations, commanded by Northern Command on behalf of the President and SecDef.
  16. At the point where Homeland Defense is in full swing, it is reasonable to assume that:
    1. Not only ALL National Guard units have been Federalized into the Army and Air Force, as well as any Federally recognized Naval Militias, but
    2. Most if not all of the entire Reserve Component has been called up for active duty (perhaps 1+ million reservists), and even a certain number of retirees have been recalled to active service (active duty retirees are very easy to recall using the authority of 10 USC 688), and
    3. Those elements of State organized militias (e.g., State Defense Forces like the Texas Maritime Regiment, the California State Guard, and many others, comprising in total several thousands of organized and at least partially trained para-military forces) that are State-only, and are not recognized by the Federal military, nor can they be “recalled to Federal service” in their State Defense Force organization, will have been called into State service under SAD orders to active duty, and
    4. The roughly 1 million law enforcement officers across the US will be in full swing, including recalling retirees and part time officers to augment their numbers, and
    5. Several millions of Federal, State, and local civil servants will be called into service to perform disaster work within their capabilities (and of course those vary, not just from person to person, but from organization to organization, and from Agency and State to State with different laws, union contracts, etc…), and
    6. The US would have invoked Article 5 of the NATO treaty, requesting as much support as required from our Allies, whether that is military, logistic, aviation, intelligence, police, EMS, hospital staffing, etc., and
    7. The US would have specifically invoked the bi-lateral treaties with Canada for both defense and civil actions, which would specifically permit Canadian military and civil personnel and organizations to move directly into the US to assist the US forces and organizations to restore order…and clearly to prevent the spread of whatever unrest is in the US from crossing into Canada.
    8. The US and Canada have a very tight working relationship, cemented by long-standing military and civil treaties, with the NATO Treaty on top of it all. There are many Canadians working in Northern Command, mostly as part of NORAD in missile and air defense, but also in other aspects. Thus, the Canadian would be well informed about our unrest, and once the Treaties for mutual support were invoked, the Canadians would begin to provide the required support for as long as it takes. After all, what are friends for? The longest undefended border in the world, and massive cross-border economic trade, and…any unrest in American is likely to involve Canada anyway, so might as well jump right in to help control it.

– – – – – – – –

And now, returning to my more historical debate…

In the US Civil War, the US Army DID operate…it existed before, during, and after, the conflict.

The US Army in the Civil War era didn’t have a reserve component, nor any National Guard. Those weren’t given full lawful, and permanent, status as Federal entities until the early 1900’s.

Today, the US Army (and each of the other Armed Forces), is made up of:

  1. The Regular Component, i.e., the full-time servicemembers who serve on active duty in that Service until their contract is up, they retire or die, or as officers serving “at the Pleasure of the President” with an “indefinite” end of service they retire, die, or decide to resign their commission.
  2. The Reserve Component, i.e., part-time (for the most part, although there are always some reservists on full-time support (FTS) orders to active duty to support the other reservists) servicemembers, many of whom are in organized reserve or National Guard units and participate in the common “one weekend a month and 2 weeks a year” requirement. There are other reservists, many hundreds of thousands of them, who are not in organized reserve units, but who are nevertheless available for recall in emergencies or war as individual servicemembers, trained manpower.
  3. And for the US Army and US Air Force, they have, in addition to the fully Federal Army Reserve and Air Force Reserve, the dual-status Army National Guard and Air Force National Guard. The National Guard (both of them) hold status first as the organized militia of their State, and as such as commissioned or enlisted into the militia of their State. They also are then recognized as members of the Federal National Guard. The Federal Government pays about 90% of all costs of the National Guard; the States pay about 10%, so they get any use of their NG units at a steep discount. If a State orders its own militia, i.e., their NG units, to State Active Duty, then the State of course pays for it all, and those units, while military, are NOT under the command or funding of the US Govt. Their State Governor is their Commander in Chief, and their State Adjutant General is their senior military commander, and so on down the line. No Federal officer may command them (unless specifically empowered to do so by agreements between the State and Federal Govts that the officer in question is specifically designated as “dual-status” commander…).

Back to the Civil War.

Since there was no Army Reserve, and there was no National Guard, who else was fighting?

Well, at the time, the “militia” of each State was a little less “organized” than today, and largely consisted of whomever volunteered to fight or participate at that time of emergency, and very little militia was maintained between emergencies…side note: this is the essence of the Second Amendment — the militia had and bore (and was trained in) weapons roughly suitable for employment during an emergency.

In theory.

In practice, many of these volunteer militias were not well-equipped or led, or even paid. Some of them were not even recognized by their State and were essentially personal guards of wealthy individuals (usually their Commander and officers), although many also were quite democratic, to the point they elected their officers and NCOs. Usually based on some previous military experience, but also based simply on leadership, charisma, or wealth.

But, the North raised many Volunteer units, mostly infantry and cavalry, but other branches were included. As the size of the Army was vastly increased, some of those Volunteers would inevitably end up joining the “Regular US Army,” but many would fight for their entire time in war as “Volunteers.”

Many “Regular” US Army veterans were detailed to help train and lead Volunteer units. One such was Lieutenant Colonel Custer, last in his class at West Point but possessed of tremendous bravery and bravado and charisma. He was quickly promoted to “Brigadier General of Volunteers,” i.e., he accepted a temporary commission at a higher rank than his permanent US Army rank.

Another side note: he was, of course, reduced back down to his permanent rank after the war, but he was allowed to remain in the very small US Army at his rank of LtCol, while allowed the honor of being addressed as “General,” in recognition of his previous Volunteer Generalcy. He died at the Little Big Horn as a Lieutenant Colonel, US Army, but is mostly known to history as “General Custer.”

So…The US Army in the Civil War grew dramatically, and all those who enlisted in, or were appointed as officer in, the US Army were automatically “Regulars” since there was no Army Reserve, and no National Guard.

Various States raised Volunteer units, and while normally they were led by their own volunteer leadership, especially after combat losses, or when their leadership was obviously failing, the US Army would detail Regular officers and NCOs to help lead those Volunteer units. But the Regulars remained regulars, and the Volunteers remained volunteers.

Today, there is a very detailed and organized method describing the Components of the Army (as detailed above), and how the Army Reserve differs from the National Guard, and how to “access” (i.e., “mobilize to active duty”) either the Reserve or National Guard, or both, for various requirements, whether in part or in full.

While the Reserves are fully Federal, and thus the States have no say over them or their personnel, the National Guard, as dual-status entities, gives States some sway over them…but not to the point of refusing to allow any or all of them to be “called to Federal service,” or “Federalized.” Several States sued the Bush Administration to attempt to prevent mobilization of some of their National Guard assets during the height of the Iraq conflict, and the Courts ruled that under the Supremacy Clause, the requirements of the Federal Govt overruled that of the States…in that specific instance, since it was clearly established long before that the National Guard was both Federal and the organized militia of a State.

And one of the President’s powers under the US Constitution is the ability to call forth the “militia” of the “several States.” Thus, the President can order any part of the “militia” as defined in US law (which includes the National Guard, and many individual citizens as well — see 10 USC 311) into Federal service…the States’ lawsuit was just tilting at a windmill like forlorn Don Quixote.