As explained in Wiki’s article United States Volunteers - Wikipedia, this was a now-defunct branch of military recruits who were distinct from both the regular army and state militia. My question is what distinguished the volunteers legally from regular army troops? Since this was long before the WW1 precedent of drafting civilians directly into the federal Army, all Army troops back then were voluntary enlistees. Presumably they could not just say “I quit” any time they felt like. The only difference I can see is that they were shorter term than regular enlistees and for reasons that escape me were authorized by special acts of Congress.
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Basically, before the creation of the organized reserve units and individual ready reserves directly attached to the branch, and the reorganization of the permanent state militias into the National Guard, the method for expanding the Army in wartime was to (a) activate the militias and (b) authorize the muster of “volunteer regiments” essentially on-the-spot, and in the early days often under direct private sponsorship, whose creation would not entail a permanent expansion of the authorized strength of the standing Regular Army (and future budget). So the idea was the Volunteers would serve only during the war or even a fixed term at the end of which there would be no ordering them to stick around. In 1847 seven regiments just went home in the middle of the Mexican War.
Never heard of such a thing. Does it really exist?
ETA — from the wiki article they (last?) existed for the Philippine-American War. 1899-1902.
Ah! This makes a lot of sense. Thanks!
They seemed to have evolved into the Army of the United States. As discussed above, this existed for the purpose of running the war. Most “regular army” officers held ranks in the Army of the United States that were considered “temporary”, with the expectation that they’d revert to their “regular” rank at war’s end.
All in all, it kind of sounds like a way for the US to screw them on their pensions 
Not anymore. The complete reorganization of the US armed forces via the 1903 Militia Act ended the practice, as noted by @JRDelirious
Nowdays, when the military needs to get around troop strength limits, they just use private contractors. It’s why, when Congress only authorized 9,000 troops in Afghanistan, there were probably twice that number of contractors. Need 300 “soldiers” to guard a base? No problem. Just pay a company to recruit 300 Ugandans for $5 a day and give them some weapons.
Thanks for that, Bear. You’re still in, right? Infantry, if memory serves? Stay safe out there.
That was more an experiment in outsourcing, seeing what fobbit jobs could be handled by civilian contractors.
So the United States Army would be the peacetime careerists, whereas the Army of the United States was everyone in uniform?
There were several reasons why calls for “volunteers” were used rather than expanding the regular army. Campaigns were often short and localized, particularly those against Native American tribes, and it was quicker and easier to simply call for short term volunteers from the closest states. It was often also easier to recruit volunteer companies, as whole communities of men would sign up with their friends and neighbors.
There was also a strong feeling in the early Republic that having a large standing army was the road to tyranny. So forces were quickly assembled and just as quickly dismissed after the need had passed, which is facilitated by having short term volunteer enlistments.
I’m sure there were special cases, but largely, yes.
You can see remnants of this system in shows like MAS*H, where they often mentioned that Col. Potter was “regular army”, which was different from the other surgeons in the unit.