The Act of July 22, 1861 allowed the President to accept Volunteers for periods of service between six months and three years, with all to be discharged at the "end of the war."
The supplementary Act of July 25, 1861 allowed for the volunteers to serve during the period of "during the war,"...
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In other words, all those volunteers, irrespective of term of service for which they were mustered-in, were subject to discharge whenever the war was concluded.
In 1863 the organization of "Veteran Volunteers" for additional "three years or the war"...
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And the President called for more volunteers for "three years or the war" but not to exceed three years...
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As to why the Union did not attempt to institute periods of service "for the duration" of the war, the closest answer would be, that the Government was satisfied that all of the citizens were subject to militia service indefinitely, State or Federal, irrespective of service in the Army, it was unnecessary.
The Volunteer troops of the Army were considered as "part of the militia system" (being derived from the Militia). And where there were no volunteers, the Government would call forth Militia to serve
as Militia when necessary. For example the President's call for 300,000 Militia for nine months in August, 1862, etc. The National draft law of 1863 aided in enrolling the whole of the Militia of the States, from which drafts of able-bodied men could more handily be made to provide soldiers for the Army where necessary.
President Lincoln reported in the State of the Union address in December, 1864 that manpower was not an issue, and that the United States could continue the struggle indefinitely...
"The important fact remains demonstrated that we have more men now than we had when the war began; that we are not exhausted nor in process of exhaustion; that we are gaining strength and may if need be maintain the contest indefinitely. This as to men. Material resources are now more complete and abundant than ever. The national resources, then, are unexhausted, and, as we believe, inexhaustible."
The national draft of 1863 was often called a "conscription" by its opposition, but it was like the system of "levies" in the Revolution (empowering the militia and the army simultaneously) rather than the European "conscription" systems which suppressed the militia of their countries.
As to opposition, in 1862, Yearn's history of the Confederate Congress states the opposition to the conscription act was limited to attempting to limit its broad scale, as in exempting the confederate militia of the states, etc. However, the lone success of the opposition was to allow the conscripted population to form volunteer units within a certain period of time, and that the conscripts of each state would only serve in State units...
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The conscription was unpopular enough that many of the Confederate congressmen were turned out in the November, 1863 election, for candidates promising reform. But the reforms made were largely to press the conscription further by the act of February, 1864, etc. The Conscript acts remained in force to the dispersion of the Confederacy, though by then unenforceable. By November, 1864 Jefferson Davis himself promoting their repeal, and laws for army unit consolidations, and a general militia law to put able-bodied men at the front, none of which were enacted.
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Previous to the conscription nearly all CSA Volunteers were in service for one year. From mid-1862 all free white male residents in the Confederacy were subject to military service for three years unless the war sooner ended.
Under the conscription acts after mid-1862, those 1-year troops, subject to the conscription act's ages of 18-35 (later expanded), who reorganized to continue in service as volunteer units, could consider their three years commencing from their initial entry; in other words two years from the date of reorganization in mid-1862. The balance subject to the conscript act could organize as three years or the war volunteers, elect their officers, etc.
If from mid-1862 any 1-year volunteer units disbanded, the men subject would have been conscripted and assigned to units of strangers, etc. Ultimately, since every man was individually a soldier in the army, the three year period was just relative to service with one unit... a discharge would subject one to conscription potentially. As Sam Watkins of the 1st Tennessee observed in his memoir, even the volunteers were aware they were ultimately conscripts too, though allowed not to serve as such during their volunteer unit's term of service.
The Conscription period, based on age, was increased to 45 in late 1862, and to 50 in early 1864. That's besides the State drafts of all 16 to 60 besides, etc.
So as the volunteers of 1861's unit time ran out into 1864, many volunteered "
for the war" as regiments etc., to forestall the confederacy breaking them up, consolidating, etc. and placing the men under officers not of their choosing. An example:
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By the last year of the war all free white men 16 to 60 were individually conscripted by the confederate conscription laws (State and Confederate), so long as those laws were enforced.
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Since every free white man was consequently declared by fiat a soldier under the direction of these acts, the only question was the nature of their service. There were those with "exemptions" from active service, and those with "details" allowing their industry to suffice as their active service, and those employed in home guards, or "local defense and special service" units, reserves units, etc., and those employed in the combat forces.
Confederate veteran George Cary Eggleston of Virginia observed of the "indefinite" period of service to which all free white men were subjected under the Confederacy...
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Apparently many foreigners in the confederacy recognized the circumstances to which they were to be subjected to the conscription act and sought the aid of their consuls where possible to get out of it...
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