case before them for
judgment. If the law include no case save those for which the
Constitution has furnished a different rule, or no case which the
Legislature has the power to govern, then the law can have no
operation. If it includes cases which the Legislature has power to
govern, and concerning which the Constitution does not prescribe a
different rule, the law governs those cases, though it may, in its
terms, attempt to include others, on which it cannot operate. In other
words, this court cannot declare void an act of Congress which
constitutionally embraces some cases, though other cases, within its
terms, are beyond the control of Congress, or beyond the reach of that
particular law. If, therefore, Congress had power to make a law
excluding slavery from this territory while under the exclusive power
of the United States, the use of the word "forever" does not
invalidate the law, so long as Congress has the exclusive legislative
power in the territory.

But it is further insisted that the treaty of 1803, between the United
States and France, by which this territory was acquired, has so
restrained the constitutional powers of Congress, that it cannot, by
law, prohibit the introduction of slavery into that part of this
territory north and west of Missouri, and north of thirty-six degrees
thirty minutes north latitude.

By a treaty with a foreign nation, the United States may rightfully
stipulate that the Congress will or will not exercise its legislative
power in some particular manner, on some particular subject. Such
promises, when made, should be voluntarily kept, with the most
scrupulous good faith. But that a treaty with a foreign nation can
deprive the Congress of any part of the legislative power conferred by
the people, so that it no longer can legislate as it was empowered by
the Constitution to do, I more than doubt.

The powers of the Government do and must remain unimpaired. The
responsibility of the Government to a foreign nation, for the exercise
of those powers, is quite another matter. That responsibility is to be
met, and justified to the foreign nation, according to the
requirements of the rules of public law; but never upon the assumption
that the United States had parted with or restricted any power of
acting according to its own free will, governed solely by its own
appreciation of its duty.

The second section of the fourth article is, "This Constitution, and
the laws of the United Sta