Re: how is the draft legal?

Article I, Section 8, Clause 12 of the Constitution authorizes Congress to "raise and support armies." Clause 13 of the same Section authorizes Congress to "provide and maintain a navy" while Clause 14 gives Congress the power to "make rules for the government and regulation of the land and naval forces." These clauses were written at a time when all governments used conscription to staff their militaries, and if the framers had intended our system to be different they would have said so.

Clauses 15 and 16 deal with the federal government's relationship with the various state militias. It does not actually require -- as your question states -- the states to have militias, but there was no reason why any state would have dispensed with a militia at the time. As fas as I know, all states still have militias in a modified form -- they are now known as the National Guard.

Re: how is the draft legal?

See Selective Draft Law Cases, 245 U.S. 366 (1918). The Court's analysis, in full, of the Thirteenth Amendment issue raised by a compulsory military

draft was the following: ''As we are unable to conceive upon what theory the exaction by government from the citizen of the performance of his supreme and noble

duty of contributing to the defense of the rights and honor of the nation, as the result of a war declared by the great representative body of the people, can be said

to be the imposition of involuntary servitude in violation of the prohibitions of the Thirteenth Amendment, we are constrained to the conclusion that the contention

to that effect is refuted by its mere statement.'' Id. at 390. While the Supreme Court has never squarely held that conscription need not be premised on a

declaration of war, indications are that the power is not constrained by the need for a formal declaration of war by ''the great representative body of the people.''

During the Vietnam War (an undeclared war) the Court, upholding a conviction for burning a draft card, declared that the power to classify and conscript

manpower for military service was ''beyond question.'' United States v. O'Brien, 391 U.S. 367, 377 (1968). See also United States v. Holmes, 387 F.2d 781,

784 (7th Cir. 1968) (''the power of Congress to raise armies and to take effective measures to preserve their efficiency, is not limited by either the Thirteenth

Amendment or the absence of a military emergency''), cert. denied 391 U.S. 956.