What was the legal methodology for accomplishing this within the framework of constitutional law?
Because if there was a legal means of doing this, it seems he would have just done it before the war and saved everybody a lot of trouble.
Article II Section 2.
The President shall be commander in chief of the army and navy of the United States, and of the militia of the several States, when called into the actual service of the United States;
Article I Section 8
To provide for calling forth the militia to execute the laws of the Union, suppress insurrections and repel invasions;
To provide for organizing, arming, and disciplining of the militia, and for governing such part of them as may be employed in the service of the United States,