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State of Tamaulipas, Mexico

The case of Fleming v. Page (1850) illustrates the same principles. The Supreme Court there held that military occupation did not make occupied districts a part of our territory under our Constitution and laws. The United States may extend its boundaries by conquest or treaty and may demand the cession of territory as the condition of peace. But this can be done only by the treaty-making power or the legislative authority, and is not a part of the power conferred upon the President by the declaration of war. His duty and his power are utterly military. As commander-in-chief he is authorized to direct the movements of the naval and military forces placed by law at his command, and to employ them in the manner he may deem most effectual to harass and conquer and subdue the enemy. He may invade the hostile country, and subject it to the sovereignty and authority of the United States. But his conquests do not enlarge the boundaries of this Union, nor extend the operation of their institutions and laws beyond the limits before assigned them by the legislative power.

It is true that when Tampico had been captured and the State of Tamaulipas subjugated, other nations were bound to regard the country, while our possession continued, as the territory of the United States and to respect it as such. For, by the laws and usages of nations, conquest is a valid title while the victor maintains exclusive possession of the conquered country. But yet it was not a part of the Union. For every nation which acquires territory by treaty or conquest holds it according to its own institutions and laws. The relation in which it stands to the United States depends not upon the law of nations, but upon our own Constitution and acts of Congress. The boundaries of the United States, as they existed before the war was declared, were not extended by the conquest, nor could they be regulated by the varying incidents of war and be enlarged or diminished as the armies on either side advanced or retreated. They remained unchanged. And every place which was out of the limits of the United States, as previously established by the political authorities of the Government, was still foreign; nor did our laws extend over it.

And in Cross v. Harrison (1853) the court observed that although Upper California was occupied by the military forces in 1846, and a government erected therein by authority of the President, still it was not a part of the United States, but conquered territory within which belligerent rights were being exercised; nor did it become part of the United States until the ratification of the treaty of peace . . . .