This provision was added in the federal Constitution to serve much the same purpose as the 'military subordinate to the civil authority' was placed in the State Constitutions.
It kept the States from encroaching on the federal authority by passing laws that said the State could coin money, make foreign treaties, etc.
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Many people don't appreciate what a brilliant document the US Constitution is. Madison what considered a Federalist in his time, but using today's standards he would be a hard-line right-winger... simply because he insisted government operate solely for the purpose for which it was intended:
The Federalist No. 39
Conformity of the Plan to Republican Principles
Independent Journal
Wednesday, January 16, 1788
[James Madison]
If we try the Constitution by its last relation to the authority by which amendments are to be made, we find it neither wholly national nor wholly federal. Were it wholly national, the supreme and ultimate authority would reside in the majority of the people of the Union; and this authority would be competent at all times, like that of a majority of every national society, to alter or abolish its established government. Were it wholly federal, on the other hand, the concurrence of each State in the Union would be essential to every alteration that would be binding on all. The mode provided by the plan of the convention is not founded on either of these principles. In requiring more than a majority, and principles. In requiring more than a majority, and particularly in computing the proportion by States, not by citizens, it departs from the national and advances towards the federal character; in rendering the concurrence of less than the whole number of States sufficient, it loses again the federal and partakes of the national character.
The proposed Constitution, therefore, [even when tested by the rules laid down by its antagonists,][1] is, in strictness, neither a national nor a federal Constitution, but a composition of both. In its foundation it is federal, not national; in the sources from which the ordinary powers of the government are drawn, it is partly federal and partly national; in the operation of these powers, it is national, not federal; in the extent of them, again, it is federal, not national; and, finally, in the authoritative mode of introducing amendments, it is neither wholly federal nor wholly national.
Learn to live with that proof -- as it cannot be logically denied.
Article 6, Clause 2
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding.
This provision was added in the federal Constitution to serve much the same purpose as the 'military subordinate to the civil authority' was placed in the State Constitutions.
It's intent is self evident from its 'bold' wording.
It kept the States from encroaching on the federal authority by passing laws that said the State could coin money, make foreign treaties, etc.
It was intended to keep any level of gov't from encroaching on our rights to life liberty and property.
-- When States insisted that it did not, civil war settled the issue militarily, and then the 14th was enacted to settle the issue constitutionally.
Obviously, some here think it is still not settled.