The Constitution has conferred upon Congress power "to declare war," "to
raise and support armies," "to provide and maintain a navy," and to call
forth the militia to "repel invasions." These high sovereign powers
necessarily involve important and responsible public duties, and among
them there is none so sacred and so imperative as that of preserving
our soil from the invasion of a foreign enemy. The Constitution has
therefore left nothing on this point to construction, but expressly
requires that "the United States shall protect each of them [the States]
against invasion." Now if a military road over our own Territories be
indispensably necessary to enable us to meet and repel the invader, it
follows as a necessary consequence not only that we possess the power,
but it is our imperative duty to construct such a road. It would be an
absurdity to invest a government with the unlimited power to make and
conduct war and at the same time deny to it the only means of reaching
and defeating the enemy at the frontier. Without such a road it is quite
evident we can not "protect" California and our Pacific possessions
"against invasion." We can not by any other means transport men and
munitions of war from the Atlantic States in sufficient time successfully
to defend these remote and distant portions of the Republic.

Experience has proved that the routes across the isthmus of Central
America are at best but a very uncertain and unreliable mode of
communication. But even if this were not the case, they would at once
be closed against us in the event of war with a naval power so much
stronger than our own as to enable it to blockade the ports at either
end of these routes. After all, therefore, we can only rely upon a
military road through our own Territories; and ever since the origin
of the Government Congress has been in the practice of appropriating
money from the public Treasury for the construction of such roads.