YES - federal law already defines the militia. Know it or male American citizens aged17-45 already are LEGALLY part of the federal militia. States can have militias too and here’s what the Constitution says about the militia...
“The Congress shall have Power To ...provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions....”
ARTICLE I, SECTION 8, CLAUSE 15
The states cannot disarm the citizens because that would remove the militia resource from the fedgov - therefore the 2nd Amendment.
Many here really do not know their American history. Militia service was not voluntary guys - at least for my ancestors on the frontier during the Revolution.
Problem: The federal government takes the invader role, and the militiamen become the insurrectionists. Gets sticky then.