Federal law requires everyone who enlists or re-enlists in the Armed Forces of the United States to take the enlistment oath.
The oath of enlistment is administered by any commissioned officer to any person enlisting or re-enlisting for a term of service into any branch of the military. The oath is traditionally performed in front of the United States Flag, and other flags, such as the state flag, military branch flag, and unit guidon may be present as well.
Armed Forces Except for the National Guard
The military oath of enlistment (re-enlistment) is fairly straightforward for most military personnel. It's administered by a superior officer, and carried out like most traditional oaths, with the officer reading the oath and the person being sworn repeating it.
I, (NAME), do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; and that I will obey the orders of the President of the United States and the orders of the officers appointed over me, according to regulations and the Uniform Code of Military Justice. So help me God.
Army or Air National Guard
The National Guard Oath of service is slightly different since Guard members also have to adhere to the Constitution of the state where they'll perform their duties.
I, (NAME), do solemnly swear (or affirm) that I will support and defend the Constitution of the United States and the State of (STATE NAME) against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; and that I will obey the orders of the President of the United States and the Governor of (STATE NAME) and the orders of the officers appointed over me, according to law and regulations. So help me God.
History of the Oath of Enlistment
During the Revolutionary War, the Continental Congress established different oaths for the enlisted men of the Continental Army. The first oath, voted on 14 June 1775, was part of the act creating the Continental Army. It read:
I (NAME) have, this day, voluntarily enlisted myself, as a soldier, in the American continental army, for one year, unless sooner discharged: And I do bind myself to conform, in all instances, to such rules and regulations, as are, or shall be, established for the government of the said Army.
The original wording was effectively replaced by Section 3, Article 1, of the Articles of War approved by Congress on 20 September 1776, which specified that the oath of enlistment read:
I (NAME) swear (or affirm) to be true to the United States of America, and to serve them honestly and faithfully against all their enemies opposers whatsoever; and to observe and obey the orders of the Continental Congress, and the orders of the Generals and officers set over me by them.
The first oath under the Constitution was approved by Act of Congress 29 September 1789 (Sec. 3, Ch. 25, 1st Congress). It applied to all commissioned officers, noncommissioned officers, and privates in the service of the United States. It came in two parts, the first of which read:
"I, A.B., do solemnly swear or affirm (as the case may be) that I will support the constitution of the United States."
The second part read:
"I, A.B., do solemnly swear or affirm (as the case may be) to bear true allegiance to the United States of America, and to serve them honestly and faithfully, against all their enemies or opposers whatsoever, and to observe and obey the orders of the President of the United States of America, and the orders of the officers appointed over me." The next section of that chapter specified that "the said troops shall be governed by the rules and articles of war, which have been established by the United States in Congress assembled, or by such rules and articles of war as may hereafter by law be established."
The 1789 enlistment oath was changed in 1960 by an amendment to Title 10, with the amendment (and current wording) becoming effective in 1962.