Analysis
From the second half of the eighteenth century onwards, international law came to distinguish between the military occupation of a country and territorial acquisition by invasion and annexation, the difference between the two being originally expounded upon by Emerich de Vattel in his opus The Law of Nations (1758). The distinction then became clear and has been recognized among the principles of international law since the end of the Napoleonic wars (circa 1820).

Sir William Blackstone, in his treatise Commentaries on the Laws of England, originally published 1765 - 1769 by the Clarendon Press at Oxford, also gave emphasis to this legal formulation. All of the territories Blackstone lists as dominions are the sovereign territory of the Crown: colonies, acquisitions and conquests, and so on.

To apply Blackstone's reasoning to the United States, we need merely substitute "federal government" for "Crown," and the meaning becomes clear.

Indeed, as early as 1828, US Supreme Court Chief Justice Marshall offered this penetrating analysis in the famous American Insurance Company case:

"The Constitution confers absolutely on the government of the Union the powers of making war and of making treaties; consequently, that government possesses the power of acquiring territory, either by conquest or by treaty."

And more explicitly, in the 1872 case of United States v. Huckabee, the Court speaking through Mr. Justice Clifford, said:

"Power to acquire territory either by conquest or treaty is vested by the Constitution in the United States. Conquered territory, however, is usually held as a mere military occupation until the fate of the nation from which it is conquered is determined .... "

Invasion and annexation later ceased to be recognized by international law and were no longer accepted as a means of territorial acquisition. The Convention respecting the Laws and Customs of War on Land (Hague IV, 1907) contained explicit provisions concerning the protection of civilians and their property in occupied territories.


Summary

In the post-Napoleonic era, in regard to territory which has been conquered by foreign military forces, the "Principle of Conquest" includes the following aspects:

• Territory may be acquired as a result of conquest by military forces, and the "conqueror" is the (principal) occupying power,
  
  
• The administration and disposition of such territory must be conducted according to the laws of war, which include the laws of military occupation,
  
  
• The territory is under military occupation, and military occupation does not transfer sovereignty,
  
  
  • Military occupation is conducted under military government,
    
    
  • Military government continues until legally supplanted by a recognized "civil government" for the area,
    
    
  • Military occupation is period of "interim (political) status."
  
  
• If there is to be a transfer of territorial sovereignty (aka "territorial cession"), then the full specifications must be given in a formal peace treaty,
  
  
• Therefore before any act of territorial cession in a peace treaty, the following actions are prohibited:
  
  
  • announcement of the annexation of, or intent to conduct an annexation of, the territory;
    
    
  • implementation of military conscription regulations over the local populace, or mobilization of local persons in an organization of a military or semi-military character;
    
    
  • institution of mass naturalization procedures, measures, or policies over the local populace.

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Application to Taiwan

All military attacks against Taiwan during the WWII period were made by US military forces, so it is clear that as a result of the Pacific War the United States has acquired Taiwan under the principle of conquest. The disposition of Taiwan must be conducted according to the laws of war.

Up until the coming into effect of the SFPT on April 28, 1952, the Commander-in-Chief dealt with the Taiwan question based on his war powers over occupied territory. Military occupation is conducted under "military government," and as "the occupying power" as spoken of in the laws of war, the United States delegated the military occupation of Taiwan to the Chinese Nationalists.

In the SFPT, Japan gave up all rights to the territorial sovereignty of Taiwan, but no recipient was named. Hence, the United States' acquirement of Taiwan was confirmed, but Taiwan remains subject to the President's war powers under United States Military Government.

Importantly, for acquired territory, "Congress shall have powers to dispose of and make all needful rules and regulations . . . . " in accordance with the territorial clause of the US Constitution (Article 4, Section 3, Clause 2).

Downes v. Bidwell, 182 U.S. 244 (1901) also confirmed that "fundamental constitutional rights" apply in all US overseas territories. The Fifth Amendment protections to life, liberty, property, and due process of law are fundamental rights under the US Constitution.

Further References and Links
Taiwan's Legal Status: Taiwan's Legal Status: An Overview of the San Francisco Peace Treaty

Areas Conquered by U.S. Military Forces and therefore under USMG Jurisdiction -- with later "new disposition" by peace treaty