What happened
The act is short and almost entirely administrative, which is what makes its two most consequential clauses easy to miss.
Section 1 raises the force. "In addition to the present military establishment, there shall be raised and organized a corps of marines," consisting of one major, four captains, sixteen first lieutenants, twelve second lieutenants, forty-eight sergeants, forty-eight corporals, thirty-two drums and fifes, and seven hundred and twenty privates.
That total is worth setting against the eighty-three Marines counted eleven days earlier. Congress authorized roughly ten times the force that existed.
Section 1 also settles what happens to the Marines already serving. The corps includes "the marines who have been enlisted, or are authorized to be raised for the naval armament," which folds the shipboard detachments of the naval armament acts into the new institution rather than starting over.
The corps could be formed into as many companies or detachments as the President directed, with a proper distribution of officers and musicians to each.
Section 2 sets pay and subsistence. A major received fifty dollars a month and four rations a day, a captain forty dollars and three rations, a first lieutenant thirty and three, a second lieutenant twenty-five and two.
The same section authorizes the President to keep enlisting until the corps is complete, to commission officers himself when the Senate is in recess, and sets enlistments at three years, subject to discharge by the President or by the ceasing or repeal of the naval armament laws.
Section 3 handles the relationship with the fleet. Detachments of the new corps are to be made "in lieu of the respective quotas of marines, which have been established or authorized for the frigates," and the President may detach and appoint officers of the Marine Corps to serve aboard as he judges necessary.
Then Section 4, which is the first of the two clauses this entry is about.
Marines "shall take the same oath, and shall be governed by the same rules and articles of war, as are prescribed for the military establishment of the United States, and by the rules for the regulation of the navy, heretofore, or which shall be established by law, according to the nature of the service in which they shall be employed."
Read the last clause slowly. Marines are governed by Army law and by Navy law, and which one applies depends on the nature of the service they are employed in at the time.
Section 4 also fixes entitlements. Marines are entitled to the same allowance in case of wounds or disabilities, according to their respective ranks, as granted by the act ascertaining and fixing the military establishment of the United States.
Section 5 exempts enlisted men, during their term of service, from all personal arrests for any debt or contract. It is not only a Marine provision: it covers seamen and the army's non-commissioned officers and musicians in the same breath, and it does not reach commissioned officers at all.
Then Section 6, which is the clause that settles the matter. "That the marine corps, established by this act, shall, at any time, be liable to do duty in the forts and garrisons of the United States, on the sea-coast, or any other duty on shore, as the President, at his discretion, shall direct."
At any time. Any other duty on shore. At his discretion.
There is no limiting principle in that sentence at all, and no requirement that the duty have anything to do with the sea.
Section 2 had already provided the machinery for it: if the Corps is ordered to do duty on shore and it becomes necessary to appoint an adjutant, paymaster, quartermaster, sergeant-major, quartermaster-sergeant, and drum and fife-major, the major or commandant of the corps may appoint them.
The act therefore contemplates shore employment twice, plainly, and puts no limit on it beyond one man's judgment.