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."
From usmilitary.about.com