It's the law of the land:
The Militia Act of 1792
Second Congress, Session I. Chapter XXVIII
Passed May 2, 1792, providing for the authority of the President to call out the Militia.
Militia Act, passed May 8, 1792, providing for the organization of the Militia follows.
Section 1. Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That whenever the United States shall be invaded, or be in imminent danger of invasion from any foreign nation or Indian tribe, it shall be lawful for the President of the United States, to call forth such number of the militia of the state or states most convenient to the place of danger or scene of action as he may judge necessary to repel such invasion, and to issue his orders for that purpose, to such officer or officers of the militia as he shall think proper; and in case of an insurrection in any state, against the government thereof, it shall be lawful for the President of the United States, on application of the legislature of such state, or of the executive (when the legislature cannot be convened) to call forth such number of the militia of any other state or states, as may be applied for, or as he may judge sufficient to suppress such insurrection.
Sec. 2. And be it further enacted, That whenever the laws of the United States shall be opposed or the execution thereof obstructed, in any state, by combinations too powerful to be suppressed by the ordinary course of judicial proceedings, or by the powers vested in the marshals by this act, the same being notified to the President of the United States, by an associate justice or the district judge, it shall be lawful for the President of the United States to call forth the militia of such state to suppress such combinations, and to cause the laws to be duly executed. And if the militia of a state, where such combinations may happen, shall refuse, or be insufficient to suppress the same, it shall be lawful for the President, if the legislature of the United States be not in session, to call forth and employ such numbers of the militia of any other state or states most convenient thereto, as may be necessary, and the use of militia, so to be called forth, may be continued, if necessary, until the expiration of thirty days after the commencement of the ensuing session.
Sec. 3. Provided always, and be it further enacted, That whenever it may be necessary, in the judgment of the President, to use the military force hereby directed to be called forth, the President shall forthwith, and previous thereto, by proclamation, command such insurgents to disperse, and retire peaceably to their respective abodes, within a limited time.
Sec. 4. And be it further enacted, That the militia employed in the service of the United States, shall receive the same pay and allowances, as the troops of the United States, who may be in service at the same time, or who were last in service, and shall be subject to the same rules and articles of war: And that no officer, non-commissioned officer or private of the militia shall be compelled to serve more than three months in any one year, nor more than in due rotation with every other able-bodied man of the same rank in the battalion to which be belongs.
Sec. 5. And be it further enacted, That every officer, non-commissioned officer or private of the militia, who shall fail to obey the orders of the President of the United States in any of the cases before recited, shall forfeit a sum not exceeding one year's pay, and not less than one month's pay, to be determined and adjudged by a court martial; and such officers shall, moreover, be liable to be cashiered by sentence of a court martial: and such non-commissioned officers and privates shall be liable to be imprisoned by the like sentence, or failure of payment of the fines adjudged against them, for the space of one calendar month for every five dollars of such fine."
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President Lincoln cites the Act in his 4/15/61 proclamation.
The Supreme Court majority opinion in the Prize Cases cites the Act.
This is new to you, but it is pretty much irrefutable.
The rebels didn't much care what the law was, because they expected to prevail at the point of a gun.
Walt