I answered a question very similar to this some time ago here.

A good case to examine regarding "go to jail or join the military" or "forced volunteering" is United States v. Catlow, conducted in the United States Army Court of Military Review and the United States Military Court of Appeals in 1973-1974. The U.S. military has always striven to obtain quality soldiers; men who are found "physically, mentally, or morally unfit" are barred from joining the military. The medical and legal standards for enlistment are sometimes relaxed in times of war, and in times of peace or after the U.S. military became all-volunteer, are often applied in a stricter manner.

Paragraph 2-6 of Army Regulation 601-210, dated May 1, 1968, provided that juveniles of enlistment age, during any point in their legal proceedings, could either continue with their proceedings as normal or, following the proper procedures, enlist voluntarily into the Regular Army for a specified term of service, after which the charges against them would be dropped;

2.) Persons who, as an alternative to further prosecution, indictment, trial, or incarceration in connection with the charges, or to further proceedings related to adjudication as a youthful offender or juvenile delinquent, are granted a release from the charges at any stage of the court proceedings on the condi­tion that they will apply for or be accepted for enlistment in the Regular Army.”

Thomas W. Catlow, born on November 14, 1951, was charged with loitering, resisting arrest, assault, and illegally carrying a concealed weapon a month before his seventeenth birthday. Catlow's home life was complicated. His parents had divorced, and his mother handed over legal guardianship of him to his uncle; however, Catlow continued to live alternately with his mother and father. In juvenile court, Catlow was informed by the judge that he could either serve five years' imprisonment for the charges, or three years in the Army. It is implied that Catlow opted for the latter, and an Army recruiter contacted him to start the induction process.

Catlow was enlisted into the Army on November 20, 1968, six days after his seventeenth birthday, and the juvenile court charges were dismissed on November 28, 1968. A problem soon arose; Catlow was a minor, and his mother, who was not his legal guardian, had signed the consent form. Catlow soon made his displeasure for military service known, and informed authorities that he thought his enlistment was not genuine. Catlow went absent without leave for a cumulative period of nearly two years, and was soon court-martialed by the Army and sentenced to six months' confinement and hard labor, forfeiture of all pay and benefits, and a dishonorable discharge. Catlow appealed his conviction.

The appellate counsel (Catlow's lawyers in the appeal case) cited the Army regulation and Catlow's enlistment circumstances as proof that Catlow's enlistment into the Army was invalid, but the Army Court of Military Review rejected their assertion in U.S. v. Catlow, 47 C.M.R. 617 (A.C.M.R., 1973), saying that Catlow, unlike many other jailbird soldiers, had avoided an actual conviction, that there were "many beneficial aspects of military service," and that his civilian legal proceedings and the official dismissal of the civilian charges against Catlow made his enlistment valid.

The United States Court of Military Appeals had a different opinion in U.S. v. Catlow, 23 U.S.C.M.A. 142, 48 C.M.R. 758 (1974). They contended that Catlow's enlistment, as a result of the policy and circumstances, was "not the product of his own volition," and that "inherent vice affected his acquisition of the status of a member of the Army." The charges against Catlow were dismissed. Another case, from 1975, United States v. Dumas, played out in a similar fashion; the defendant was not given proper consent to join the military by his legal guardian, and so his enlistment was invalidated.

The U.S. military soon moved to re-write their regulations, and the choice of "go to jail or join the military" was done away with. it was still a problem in the Army as late as December 1977; many civilian courts had yet to be informed of the policy changes, which exasperated military authorities.

All U.S. military branches now explicitly prohibit a person from enlisting if they are released from civilian legal proceedings under the stipulation that they join the military. Military recruiters are also prohibited from appearing in court on the behalf of any applicant, and may not offer advice to, or help to an unqualified applicant to enlist.

United States Air Force:

Air Force Recruiting Regulation, AETCI 36-2002, table 1-1, lines 7 and 8;

[An applicant is ineligible for enlistment if they are] released from restraint, or civil suit, or charges on the condition of entering military service, if the restraint, civil suit, or criminal charges would be reinstated if the applicant does not enter military service.

United States Army:

Army Regulation 601-210, paragraph 4-8b;

[Any] applicant who, as a condition for any civil conviction or adverse disposition or any other reason through a civil or criminal court, is ordered or subjected to a sentence that implies or imposes enlistment into the Armed Forces of the United States, is not eligible for enlistment.

Paragraph 4-32a of the same regulation states the following;

Waiver is not authorized if a criminal or juvenile court charge is pending or if such a charge was dismissed or dropped at any stage of the court proceedings on condition that the offender enlists in a military service.

United States Coast Guard:

Coast Guard Recruiting Manual, M1100.2D, Table 2-A;

An application may be denied when, based on articulable facts, it is determined that accession would not be in the best interest of the Coast Guard.

United States Marine Corps:

Marine Corps Recruiting Regulation, MCO P1100.72B, Chapter 3, Section 2, Part H, Paragraph 12;

Applicants may not enlist as an alternative to criminal prosecution, indictment, incarceration, parole, probation, or another punitive sentence. They are ineligible for enlistment until the original assigned sentence would have been completed.

United States Navy:

Navy Recruiting Manual-Enlisted CNRC1130.8H, Section 02083(l):

Applicants may not enlist as an alternative to criminal prosecution, indictment, incarceration, parole, probation, or another punitive sentence. They are ineligible for enlistment until the original assigned sentence would have been completed.

Sources:

"Forced Enlistments Plague the Army." American Bar Association Journal 63, no. 12 (December 1977): 1699.

Milburn, Travis. “Exploring Military Service as an Alternative Sanction: Evidence From Inmates' Perspectives.” Master's thesis, Eastern Kentucky University, 2012.

Schogol, Jeff. “Judge said Army or jail, but military doesn’t want him.” Stars and Stripes (Washington, D.C.), Feb. 3, 2006.

United States. United States Army. Department of the Army Pamphlet 27-50-19 The Army Lawyer, July 1974. Washington: United States Department of the Army, 1974.