1. According to the express words of the Constitution, it is one of the powers that the states are forbidden to exercise without the consent of Congress.
2. It is incompatible and inconsistent with the powers conferred on the federal government.
The first clause of the tenth section of the first article of the Constitution, among other limitations of state power, declares that "no state shall enter into any treaty, alliance, or confederation;" the second clause of the same section, among other things, declares that no state without the consent of Congress, shall "enter into any agreement or compact with another state, or with a foreign power."
We have extracted only those parts of the section that are material to the present inquiry. The section consists of but two paragraphs, and is employed altogether in restrictions upon the powers of the states. In the first paragraph, the limitations are absolute and unconditional; in the second, the forbidden powers may be exercised with the consent of Congress, and it is in the second paragraph that the restrictions are found which apply to the case now before us.
In expounding the Constitution of the United States, must have its due force and appropriate meaning, for it is evident from the whole instrument that no word was unnecessarily used or needlessly added. The many discussions which have taken place upon the construction of the Constitution have proved the correctness of this proposition and shown the high talent, the caution, and the foresight of the illustrious men who framed it. Every word appears to have been weighed with the utmost deliberation, and its force and effect to have been fully understood. No word in the instrument, therefore, can be rejected as superfluous or unmeaning, and this principle of construction applies with peculiar force to the two clauses of the tenth section of the first article, of which we are now speaking, because the whole of this short section is directed to the same subject -- that is to say it is employed altogether in enumerating the rights surrendered by the states; and this is done with so much clearness and brevity that we cannot for a moment believe that a single superfluous word was used, or words which meant merely the same thing. When, therefore, the second clause declares that no state shall enter into "any agreement or compact" with a foreign power without the assent of Congress, the words "agreement" and "compact" cannot be construed as synonymous with one another; and still less can either of them be held to mean the same thing with the word "treaty" in the preceding clause, into which the states are positively and unconditionally forbidden to enter, and which even the consent of Congress could not authorize.
But the question does not rest upon the prohibition to enter into a treaty. In the very next clause of the Constitution, the states are forbidden to enter into any "agreement" or "compact" with a foreign nation, and as these words could not have been idly or superfluously
used by the framers of the Constitution, they cannot be construed to mean the same thing with the word "treaty." They evidently mean something more, and were designed to make the prohibition more comprehensive.
After reading these extracts, we can be at no loss to comprehend the intention of the framers of the Constitution in using all these words, "treaty," "compact," "agreement." The word "agreement," does not necessarily import any direct and express stipulation; nor is it necessary that it should be in writing. If there is a verbal understanding to which both parties have assented and upon which both are acting, it is an "agreement." And the use of all of these terms, "treaty," "agreement," "compact," show that it was the intention of the framers of the Constitution to use the broadest and most comprehensive terms, and that they anxiously desired to cut off all connection or communication between a state and a foreign power, and we shall fail to execute that evident intention unless we give to the word "agreement" its most extended signification, and so apply it as to prohibit every agreement, written or verbal, formal or informal, positive or implied, by the mutual understanding of the parties.