Exodus 20 (Young's Literal Translation)
13 `Thou dost not murder.
17 `Thou dost not desire the house of thy neighbour, thou dost not desire the wife of thy neighbour, or his man-servant, or his handmaid, or his ox, or his ass, or anything which is thy neighbour's.'

Nature of war.

"War, in law, is not a mere contest of physical forces, on however large a scale.  War in the legal sense is the state of nations among whom there is an interruption of all pacific relations and a general contestation of arms by authority of the several sovereigns; it is not a mere contest of force, but must be an armed struggle carried on between two political bodies each of which exercises de facto authority over persons within a definite territory, and its existence is determined by the authorized political department of the government.  So, lawful war can never exist without the actual concurrence of the war-making power, but may exist prior to any contest of the armed forces." (30 CJS War, ¤ 1 Definition and Nature.)

"War is an armed struggle or contest by force carried on for any purpose between two or more nations or states exercising at least de facto authority over persons within a given territory and commanding an army prepared to observe the ordinary laws of war.   War also may be defined as consisting of the exercise of force by bodies politic against each other and under the authority of their respective governments with a purpose of coercion, and is that state in which a nation prosecutes its rights or its claims by force of arms." (78 Am Jur 2d, War, ¤ 1, Generally.)

"Every contention by force between two nations, in external matters under the authority of their respective governments, is not only war, but public war.  If war is formally declared, it is called "solemn" war and is of the perfect kind all the members of the nation declaring war are authorized to commit hostilities against all the members of the other, in every place and under every circumstance. In such a war all the members act under a general authority and all the rights and consequences of war attach to their condition.  But hostilities may subsist between two nations, more confined in nature and extent, being limited as to places, persons, and things; and this is more properly termed "imperfect" war, because not solemn, and because those who are authorized to commit hostilities act under special authority, and can go no farther than to the extent of their commission.  Still, however, it is a public war, because it is an external contention by force between some of the members of the two nations, authorized by the legitimate powers.  It is a war between the two nations, although all the members are not authorized to commit hostilities as in a solemn war where the government restrains the general power.  The Indian wars were of this latter class." (78 Am Jur 2d, War, ¤ 4, Existence of war.)

"War may exist without declaration on either side. … It is, however, essential that some form of public act, proceeding directly from the competent source, shall announce to the people at home their new relations and duties flowing out of the state of war."
"To warrant the destruction of property, or the taking of life on the ground of public war, it must be what it called "lawful" war by the law of nations, a thing which can never exist without the expressed concurrence of the war-making power. In any other war no belligerent rights can be acquired.  All captures, all destruction of property, must be illegal, and the taking of life a crime.  Short of this, war cannot be carried into an enemy's country, for the simple reason that there is no war to carry there, and no enemy against whom it can be exerted.  The nation denouncing war must be explicit.  "This makes it," says Vattel, "formal, and so lawful."  "But nothing of this kind," says he, "is the case in informal, illegitimate war, which is more properly call 'depredation.' A nation attacked by enemies, without the sanction of a public war, is not under any obligation to observe towards them the rules of formal warfare.  She may treat them as robbers."   "Such unauthorized volunteers in violence," says Blackstone, "are not ranked among open enemies, but are treated like pirates and robbers." (78 Am Jur 2d, War, ¤ 5, Initiation; necessity of formal declaration.)

"In the United States the power to declare war resides in Congress alone, and the President has no such power unless it has lawfully been given him by Congress, although he has power, without any declaration of war by Congress, to repel invasions, and in so doing may carry hostilities into the invader's own country." (30 CJS War, ¤ 2 Power to Make War.)

"The right to make war belongs, of course, to the sovereign power. Under our Constitution, Congress, and Congress alone, has the power to declare a national or foreign war.  This power is conferred not for the purposes of aggression or aggrandizement, but to vindicate the rights of the government and those of its citizens.  Although the President, in whom the executive power is vested, is the Commander in Chief of the Army and Navy of the United States, the President has not legal power to initiate or declare war, against either a foreign nation or a domestic state.  The President, however, has the power to recognize the existence of a state of war, and if war upon this country is made by invasion of a foreign nation, the President is not only authorized, but bound, to resist force by force; he is bound to accept the challenge without waiting for special legislative authority.  By acts of Congress he is authorized in case of invasion by foreign nations, and to suppress insurrection against the government of a state or of the United States.
"Since war cannot lawfully be commenced on the part of the United States without an act of Congress, such an act is a formal official notice to all the world, and equivalent to the most solemn declaration.  But while, as between the United States and other civilized nations, an act of Congress is necessary to a formal declaration of war, no such act was necessary to constitute a state of war with an Indian tribe." (78 Am Jur 2d, War, ¤ 7, Power to make or declare war.)

"[T]he Korean conflict, it has been held … was not a "war" within what may be termed the constitutional or legal sense of that word, but … it is clear that there was war in fact in Korea." (30 CJS War, ¤ 1 Definition and Nature.)

"Although Congress, in certain enactments, recognized the operation of United States military forces in Korea and appropriated funds for their support, the conflict was not to be a "war" in the constitutional or legal sense in the absence of a declaration of war by congress. (Beley v. Pennsylvania Mut. Life Ins. Co. 373 Pa 231, 95 A2d 202, 36 ALR2d 996, cert den 346 US 820, 98 L Ed 346, 74 S Ct 34.) [78 Am Jur 2d. ¤ 5, fn 31.]

U. S. Congress did not legislate for theocratic war.
From annotations on 50 USCA App. 451 and 456 regarding conscientious objectors.

"Thus it is seen "war in any form" was construed to include theocratic wars.  However, the Court does not believe that Congress intended such an unconscionable interpretation to be placed on this phrase. War, as the term is commonly used, is a conflict by force between two or more nations; it is a conflict of violence by one politically organized body seeking to overcome or overthrow another political entity.   It is patent that Congress was legislating in regard to this type of struggle, and was not concerned with wars carried on by the command or direction of God, i.e. theocratic wars. Taffs v. United States, 8 Cir., 1953, 208 F.2d 329; United States v. Hartman, 2 Cir., 1954, 209 F.2d 366.  See also United States v. Hagaman, 3 Cir., 213 F.2d 86.   It is clear that Congress intended by this section to exempt from service in the armed forces those persons who were conscientiously opposed by reason of religious training and belief to any form of participation in a flesh-and-blood conflict between nations." (UNITED STATES v. BORTIK, 122 F.Supp 225, 227.) [50 USCA App. ¤ 451, nts. 5,6.]

"The government would have us construe the words "war in any form" to include the theocratic warfare discussed in the two articles appearing in the February 1, 1951, issue of "The Watchtower," the argument being that since appellant believed in and approved of theocratic warfare, he was not opposed to "participation in war in any form."  However, we do not believe that Congress intended such an unreasonable construction to be placed on this phrase.  War, as the word is commonly understood, is a contest between two or more nations; it is the state or fact of one politically organized body exerting violence or force against another for the purpose of overcoming or overthrowing it.  It is clear to us that Congress was legislating in regard to this type of conflict, and was not concerned with scriptural or theocratic wars, considered as carried on by the command and direction of God.  The phrase "in any form" obviously modifies not the word "war" but the word "participation." Taffs v. United States, supra.  War generally speaking, has only one form, namely a struggle between opposing forces; whereas a person's participation therein may be in a variety of forms.  We are convinced that Congress intended by this section to exempt from service in the armed forces those persons of religious training and belief to any form of participation in a flesh-and-blood war between political entities." (UNITED STATES v. HARTMAN, 209 F.2d 366, 370.)

"Can it be said that a belief in a right of self-defense or a belief in the righteousness of the theocratic wars of the Old Testament place the appellant outside the exemption of the statute?
This question has been considered in Taffs v. United States, 8 Cir., 208 F.2d 329; United States v. Hartman, 2 Cir., 209 F.2d 366, and Annett v. United States, 10 Cir., 205 F.2d 689.
We do not think that Congress in using the words "participation in war in any form" contemplated the sort of self defense of an individual or his family or the theocratic warfare described in the Bible as coming within the meaning of that phrase.   See in particular what was stated with respect to this in United States v. Hartman, supra, 209 F.2d at page 370." (TOMLINSON v. UNITED STATES, 216 F.2d 12.)

"A person's willingness to use force in self-defense is not a valid objection to denial of selective service classification as a conscientious objector, where other evidence of such person's opposition to participation in war because of religious belief is undisputed Universal Military Training and Service Act, ¤ 6(j), 50 U.S.C.A.Appendix, ¤ 456(j)." (TAFFS v. UNITED STATES, 208 F.2d 329, hn 2.)

"Under Universal Military Training and Service Act provision that nothing shall be construed to require any person to be subject to combatant training and service in armed forces who, by reason of religious training and belief, is conscientiously opposed to participation in war in any form, Congress intended to exempt those persons from serving in armed forces whose religious beliefs were opposed to any form of participation in flesh and blood war between nations and Congress was not legislating in regard to theocratic wars. Universal Military Training and Service Act, ¤¤ 6(j), 12, 50 U.S.C.A.Appendix, ¤¤ 456(j), 462." (TAFFS v. UNITED STATES, 208 F.2d 329, hn 3.)

"In prosecution for failure to submit to induction into armed forces of the United States, where there was evidence that defendant because of religious belief was opposed to service in armed forces but that defendant believed in and approved of theocratic wars, whether a certain war was theocratic or not was a matter of religious belief into which court was forbidden to delve." (TAFFS v. UNITED STATES, 208 F.2d 329, hn 4.) "Whether a certain war is theocratic or not is a matter of religious belief into which we are forbidden to delve." (TAFFS v. UNITED STATES, 208 F.2d 329, 331.)

Generally.

Every contention by force between two nations, in external matters, under the authority of their respective governments, is not only war, but public war. Arce v. State, L.R.A.1918E, 358, 80 Tex. Crim. Rep. 331, 202 S. W. 951.

It is not necessary to constitute war, that both parties should be ackowledged as independent nations or sovereign states. A war may exist where one of the belligerents claims sovereign rights as against the other. Arce v. State, L.R.A.1918E, 358, 80 Tex. Crim. Rep. 331, 202 S. W. 951.

Martial law.

Martial law may be instituted, in case of invasion, insurrection, or riot, in a magisterial district of a county, and offenders therein punished by the military commission, notwithstanding the civil courts are open and sitting in other portions of the county. State ex rel. Mays v. Brown, 45 L.R.A.(N.S.) 996, 71 W. Va. 519, 77 S. E. 243.

A state of war having been declared in any part of the state on an occasion of insurrection, the war power of the state in the form of military rule, defined by the usages of nations, prevails in the territory subject to the proclamation, excluding the civil powers is to offesnes, if the executive so order, while the peace powers of government under civil law prevail elsewhere. Ex parte Jones, 45 L.R.A.(N.S.) 1030, 71 W. Va. 567, 77 S. E. 1029.

Alien enemies.

An alien enemy is one who owes allegiance to an adverse belligerant nation. Dorsey v. Brigham, 42 L.R.A. 809, 177 Ill. 250, 52 N. E. 303.

Alien enemies have no rights and no privileges, unless by special favor , during time of war, and there is nothing in the Constitution or laws of the United States which in any way has changed the common-law rule, or restricted the power of Congress to enact the Alien Enemy Law. DeLacey v. United States, L.R.A. 1918E, 1011, 161 C. C. A. 535, 249 Fed. 625.

The constitutional provisions against depriving one of liberty without due process of law do not apply to an alien enemy. DeLacey v. United States, L.R.A. 1918E, 1011, 161 C. C. A. 535, 249 Fed. 625.

A resident citizen of Austria-Hungary whose business is fishing in the public waters of a state is entitled to a license to pursue such business as authorized by the state of his residence, under the proclamation of December 11, 1917, that so long as citizens of that government shall conduct themselves in accordance with law they shall be undisturbed in the peaceful prosecution of their lives and occupations. State ex rel. Constanti v. Darwin, L.R.A. 1918F, 1012, 102 Wash. 402, 173 Pac. 29.

--trading with.

War prohibits all trading with the enemy except with the Royal license, and dissolves all contracts which involve such trading. Zinc Corp. v. Hirsch, L.R.A. 1917C, 650, 1 K. B. (1916) 541.

--actions and proceedings by and against.

--confiscation of property of.

The property of alien enemies is subject to confiscation by the Crown in virtue of the Royal prerogative. Halsey v. Lowenfeld, L.R.A. 1917C, 644, 2 K. B. (1916) 707.

Military tribunals.

The authorized application of martial law to territory in a state of war includes the power to appoint a military commission for the trial and punishment of offenses within such territory. State ex rel. Mays v. Brown, 45 L.R.A.(N.S.) 996, 71 W. Va. 519, 77 S. E. 243.

Acts committed in the short interim between two military occupations of a territory for the suppression of insurrection and riotous uprisings, and such in their general nature as those characterizing the uprising, are punishable by the military commission within the territory and period of military occupation. State ex rel. Mays v. Brown, 45 L.R.A.(N.S.) 996, 71 W. Va. 519, 77 S. E. 243.

The jurisdiction of a military commission is not confined to military persons, but extends also to citizens. Ex parte Jones, 45 L.R.A.(N.S.) 1030, 71 W. Va. 567, 77 S. E. 1029.

Wheaton's International Law, Fifth Edition (1916)
Rights of War as Between Enemies--Land Warfare (p. 521)

"... The local inhabitants do not owe the occupant even temporary allegiance; and the national character of the locality is not legally changed.
"This view has long been adopted by British Courts. Thus, is The Gerasimo (1857) (m), the Privy Counsil pointed out that in order to convert a friendly or nuetral territory into enemy territory, it was not sufficient that the territory in question should be under hostile occupation and subjected to the control of a hostile Power; some additional proceeding was necessary, e.g., cession or conquest, whereby the territory was incorporated with and made part of the dominions of the invader (n). This priciple was adopted not only in the British Prize Courts, but also in the Courts of common law (o). Lord Stowell emphasised the distinction between a hostile occupation and possession clothed with a legal right by cession or conquest, or confirmed by lapse of time (p). It was held, too, in the French Courts (q) that a crime committed by a Frenchman in Spanish territory which was at the time occupied by the French forces was a crime committed in foreign territory.
"Some decisions of the American Courts adopt a different point of view. Thus the Supeme Court held that when a portion the American Union is occupied by a public enemy, that portion is to be deemed a foreign country in respect of revenue laws, and that goods imported into it are not imported into the Union (r). On the other hand, when forces of the Union occupy a foreign territory, such territory comes under the sovereignty of the Union, but does not become part of the United States, although foreign nations are bound to regard it as such. It is to be governed by military law, as regulated by public law. This is due to the fact that the Presidnet has power to make war, and subject the enemy's country, but only in a military sense. But he has no power to enlarge the boundaries of the Union--which can be done only by congress, the treaty-making power (s). ..."

(m) 11 Moo. P. C. 88.
(n) Cf. The Fama (1804), 5 C. Rob. 115; The Manilla, 1 Edw. 1; The Santa Anna, 1 Edw. 180.
(o) Donaldson v. Thompson (1808), 1 Camp. 429; Hagedorn v. Bell, 1 M. & S. 450.
(p) The Bolletta, 1 Edw. 171.
(q) Villasseque's Case (1818), Ortolan, i. 324.
(r) U. S. v. Rice (1819), 4 Wheaton, 246.
(s) Fleming v. Page (1850), 9 Howard, 615; Neeley v. Henkel (1900), 180 U. S. Rep. 109; But see The Circassian (1864), 2 Wall. 135.

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Penalty for not registering for selective service:

http://www4.law.cornell.edu/uscode/50X/462.html

(f)
(1) Except as provided in subsection (g), any person who is required under section 3 (section 453 of this Appendix) to present himself for and submit to registration under such section and fails to do so in accordance with any proclamation issued under such section, or in accordance with any rule or regulation issued under such section, shall be ineligible for any form of assistance or benefit provided under title IV of the Higher Education Act of 1965 (20 U.S.C. 1070 et seq.; 42 U.S.C. 2751 et seq.).
(2) * * * [more]

Another writer on the subject, in a letter to the Director to Selective Service:
http://bradbva.home.mindspring.com/freedom/sss.htm

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