The Hague Conventions of 1907 specify that "territory is considered occupied when it is actually placed under the authority of the hostile army."

On page 21 of his book Military Government and Martial Law, United States Army Brigadier General William E. Birkhimer specifies that:
The US Constitution has placed no limit upon the war powers of the government, but they are regulated and limited by the laws of war. One of these powers is the right to institute military governments.

The U.S. Supreme Court case of Ex Parte Milligan, 71 U.S. 2 (1866), is an authoritative reference. The judges held that:
MILITARY GOVERNMENT is exercised in time of foreign war without the boundaries of the United States, or in time of rebellion and civil war within states or districts occupied by rebels treated as belligerents; superseding, as far as may be deemed expedient, the local law, and exercised by the military commander under the direction of the President, with the express or implied sanction of Congress . . .

More simply, "military government." is the form of administration by which an occupying power exercises governmental authority over occupied territory. Any area under military government jurisdiction is considered to be occupied territory.

Although it seems reasonable to say that the local inhabitants have the right to know that they are being occupied, nevertheless legally speaking, there does not have to be a formal announcement of the beginning of "military government," nor is there any requirement of a specific number of people to be in place, or "on site" before military government can be said to have commenced.

See Birkhimer, p. 25 - 26
No proclamation of the part of the victorious commander is necessary to the lawful inauguration and enforcement of military government. That government results from the fact that the former sovereignty is ousted, and the opposing army how has control. {22}Yet the issuing such proclamation is useful as publishing to all living in the district occupied those rules of conduct which will govern the conqueror in the exercise of his authority. Wellington, indeed, as previously mentioned, said that the commander is bound to lay down distinctly the rules according to which his will is to be carried out. But the laws of war do not imperatively require this, and in very many instances it is not done. When it is not, the mere fact that the country is militarily occupied by the enemy is deemed sufficient notification to all concerned that the regular has been supplanted by a military government.
Reference: {22} Instructions for Armies in the Field, Gi O. 100, A.G. 0., 1863.

Military Government includes civil administration of military government for interim cessions, which is commonly composed of both civil and military components. Technically speaking, military government is used as an interim and provisional government of undetermined cessions, and especially for "foreign territory" under control by conquest; and while it is not martial law but it can be indefinite; hence the some persons regard military government as the international law equivalent of "martial law."

Military Government and Martial Law

The distinction between these two types of military jurisdiction is clarified by Birkhimer, see p. 307
Martial law is that rule which is established when civil authority in the community is made subordinate to military, either in repelling invasion or when the ordinary administration of the laws fails to secure the proper objects of government.
The following two quotations are on Birkhimer, p. 1
Military jurisdiction is treated in the following pages in its two branches of Military Government and Martial Law. The former is exercised over enemy territory; the latter over loyal territory of the State enforcing it.
. . . The distinction is important. Military government is thus placed within the domain of international law, its rules the laws of war, while martial law is within the cognizance of municipal law.

Clarifications about Military Government, Part 1

Birkhimer, p. 16
Military Government is that which is established by a commander over occupied enemy territory. To entitle it to recognition it is necessary that the authority of the State to which the territory permanently belongs should have ceased there to be exercised.
Birkhimer, p. 26
Military government continues till legally supplanted
New Mexico was not only conquered, but remained thereafter under the dominion of the United States. The provisional government established therein ordained laws and adopted a judicial system suited to the needs of the country. The Supreme Court of the United States held that these laws and this system legally might remain in force after the termination of the war and until modified either by the direct legislation of Congress or by the territorial government established by its authority. We have had the same experiences in Cuba, Porto Rico, and the Philippines.
Birkhimer, p. 1
Moreover, military government may be exercised not only during the time that war is flagrant, but down to the period when it comports with the policy of the dominant power to establish civil jurisdiction.
An additional explanation regarding the nature of military government is given in Winthrop's opus, as quoted in the US Supreme Court case of Madsen v. Kinsella, 343 U.S. 341 (argued Jan. 8, 1952, decided April 28, 1952):
In speaking of the nature of military government, Colonel William Winthrop, in his authoritative work on Military Law and Precedents (second edition, 1920 reprint), says on page 800: "Military government . . . is an exercise of sovereignty, and as such dominates the country which is its theatre in all the branches of administration. Whether administered by officers of the army of the belligerent, or by civilians left in office or appointed by him for the purpose, it is the government of and for all the inhabitants, native or foreign, wholly superseding the local law and civil authority except in so far as the same may be permitted by him to subsist . . . . The local laws and ordinances may be left in force, and in general should be, subject however to their being in whole or in part suspended and others substituted in their stead -- in the discretion of the governing authority."

Clarifications about Military Government, Part 2

The Occupying Power
The terminology of "the occupying power" as spoken of in the laws of war is most properly rendered as "the principal occupying power," or alternatively as "the (principal) occupying power." This is because the law of agency is always available.

Explanatory Notes: When the administrative authority for the military occupation of particular areas is delegated to other troops, a "principal -- agent" relationship is in effect.

The conqueror is the (principal) occupying power. This is the clear precedent as established in California, Utah, Nevada, Puerto Rico, Guam, the Philippines, Cuba, etc.

The Political Status of Occupied Territory

Military occupation is period of "interim (political) status." The occupied territory is said to be "in interim status under the law of occupation." Since the territory has not reached a final (political) status, it is a sub-sovereign entity.

Explanatory Notes: The status of the territory can also be described as "undetermined," or as an "independent customs area."

The Significance of Specifying a "Receiving Country" for a Territorial Cession in a Peace Treaty

The designation of a "receiving country" for a territorial cession in a peace treaty means that the Legislative Branch of the "receiving country" is authorized to pass legislation to establish civil government in the territory.

Explanatory Notes: (A) It should be recognized that at the point of cession, the territory is actually being ceded to the military government of the principal occupying power. (B) Before the receiving country's civil government begins operations, the territory remains under the jurisdiction of the principal occupying power and in "interim status." (C) Without the appropriate specifications in a treaty, there is no authorization for any "country" to establish civil government in the territory, and military government (of the principal occupying power) continues until legally supplanted.

Occupied territory is "foreign territory"

See Downes v. Bidwell, 182 U.S. 244 (1901)
So long as Congress has not incorporated the territory into the United States, neither military occupation nor cession by treaty makes the conquered territory domestic territory, in the sense of the revenue laws; but those laws concerning "foreign countries" remain applicable to the conquered territory until changed by Congress. Such was the unanimous opinion of this court, as declared by Chief Justice Taney in Fleming v. Page, (1850) 9 How. 603, 617, 13 L. ed. 276, 281.

See Birkhimer, p. 1
From a belligerent point of view, therefore, the theatre of military government is necessarily foreign territory.
See Birkhimer, p. 21
Military government foreign territory
The erection of such governments over the persons and territory of a public enemy is an act of war; is in fact the exercise of hostilities without the use of unnecessary force. It derives its authority from the customs of war, and not the municipal law.
See Birkhimer, p. 43
Important consequences, occupied territory regarded as foreign
Important consequences result from the rule that territory under military government is considered foreign.

Hence, the statement that
Taiwan an occupied territory of the United States.
is indeed equivalent to saying that
Taiwan is under the jurisdiction of the United States Military Government (USMG).
but is not equivalent to saying that
Taiwan is part of the United States.

Based on the decision in DeLima v. Bidwell 182 U.S. 1 (1901), Taiwan may be said to be "foreign territory under the dominion of the United States."

Also See FM 27-10

362. Necessity for Military Government

Military government is the form of administration by which an occupying power exercises governmental authority over occupied territory. The necessity for such government arises from the failure or inability of the legitimate government to exercise its functions on account of the military occupation, or the undesirability of allowing it to do so. (See para. 12, which discusses military government, and para. 354, dealing with civil affairs administration.)

363. Duty to Restore and Maintain Public Order

The authority of the legitimate power having in fact passed into the hands of the occupant, the latter shall take all the measures in his power to restore, and ensure, as far as possible, public order and safety, while respecting, unless absolutely prevented, the laws in force in the country. (HR, art. 43.)

368. Nature of Government

It is immaterial whether the government over an enemy's territory consists in a military or civil or mixed administration. Its character is the same and the source of its authority the same. It is a government imposed by force, and the legality of its acts is determined by the law of war.

Military Government and the Occupation of Taiwan

Military occupation is conducted under military government, and for Taiwan the United States has delegated the military occupation of Taiwan to the ROC. The ROC as an agent for USMG on Taiwan is fully apparent under the plain language of the SFPT, and relevant analysis is given as follows:
  • The ROC did not sign the SFPT, and cannot be considered one of the Allied Powers under the treaty,
  • hence the ROC on Taiwan does not represent the "Allied Powers,"
  • nor is it an organ established by the Allies,
  • . . . but rather the ROC is an agent of USMG, and
  • the SFPT fully recognizes the USA as the principal occupying power,
  • with USMG jurisdiction over the territory of Taiwan.
United States Military Government (USMG) in Taiwan has begun as of October 25, 1945, with the surrender of Japanese troops.

Importantly, the military government of the principal occupying power does not end with the coming into force of the peace treaty, but continues until legally supplanted. Specifications for the continuance of USMG jurisdiction over Taiwan is given in the SFPT:

Article 4
(b) Japan recognizes the validity of dispositions of property of Japan and Japanese nationals made by or pursuant to directives of the United States Military Government in any of the areas referred to in Articles 2 and 3.

Additional Webpages of Interest

Introduction and Outline for Modern Taiwanese History

Unincorporated territory under USMG

Three Insular Cases and the Taiwan Status

Military Government and Martial Law

by William E. Birkhimer
Kansas City, Missouri, Franklin Hudson Publishing Co.
third edition, revised (1914)

This book may be downloaded from
Internet Archive

FM 27-10  The Law of Land Warfare

Dept. of the Army
Washington, D.C.
revised   July 15, 1976

An HTML version of this Field Manual is available at
FM 27-10