It may be said that although the authority to exercise exclusive legislation in certain cases within the States with their consent may be considered as a prohibition to Congress to exercise like exclusive legislation in any other case, although their consent should be granted, it does not prohibit the exercise of such jurisdiction or power within a State as would be competent to all the purposes of internal improvement.  I can conceive no ground on which the idea of such a power over any part of the territory of a State can be inferred from the power to declare war.  There never can be an occasion for jurisdiction for military purposes except in fortifications, dockyards, and the like places.  If the soldiers are in the field or are quartered in garrisons without the fortifications, the civil authority must prevail where they are.  The government of the troops by martial law is not affected by it.  In war, when the forces are increased and the movement is on a greater scale, consequences follow which are inseparable from the exigencies of the state.  More freedom of action and a wider range of power in the military commanders, to be exercised on their own responsibility, may be necessary to the public safety; but even here the civil authority of the State never ceases to operate.  It is also exclusive for all civil purposes.

Whether any power short of that stated would be adequate to the purposes of internal improvement is denied.  In the case of territory one government must prevail for all the purposes intended by the grant.  The jurisdiction of the United States might be modified in such manner as to admit that of the State in all cases and for all purposes not necessary to the execution of the proposed power; but the right of the General Government must be complete for all the purposes above stated.  It must extend to the seizure and condemnation of the property, if necessary; to the punishment of offenders for injuries to the roads and canals; to the establishment and enforcement of tolls, etc.  It must be a complete right to the extent above stated or it will be of no avail.  That right does not exist.

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.