The reasons which operate in favor of the right of exclusive legislation in forts, dockyards, etc., do not apply to any other places. The safety of such works and of the cities which they are intended to defend, and even of whole communities, may sometimes depend on it. If spies are admitted within them in time of war, they might communicate intelligence to the enemy which might be fatal. All nations surround such works with high walls and keep their gates shut. Even here, however, three important conditions are indispensable to such exclusive legislation: First, the ground must be requisite for and be applied to those purposes; second, it must be purchased; third, it must be purchased by the consent of the State in which it may be. When we find that so much care has been taken to protect the sovereignty of the States over the territory within their respective limits, admitting that of the United States over such small portions and for such special and important purposes only, the conclusion is irresistible not only that the power necessary for internal improvements has not been granted, but that it has been clearly prohibited. I come next to the right to regulate commerce, the third source from whence the right to make internal improvements is claimed. It is expressed in the following words: "Congress shall have power to regulate commerce with foreign nations and among the several States and with the Indian tribes." The reasoning applicable to the preceding claims is equally so to this. The mischief complained of was that this power could not be exercised with advantage by the individual States, and the object was to transfer it to the United States. The sense in which the power was understood and exercised by the States was doubtless that in which it was transferred to the United States. The policy was the same as to three branches of this grant, and it is scarcely possible to separate the two first from each other in any view which may be taken of the subject. The last, relating to the Indian tribes, is of a nature distinct from the others for reasons too well known to require explanation. Commerce between independent powers or communities is universally regulated by duties and imposts. It was so regulated by the States before the adoption of this Constitution equally in respect to each other and to foreign powers. The goods and vessels employed in the trade are the only subjects of regulation. It can act on none other. A power, then, to impose such duties and imposts in regard to foreign nations and to prevent any on the trade between the States was the only power granted.