Not sure if anyone has ever heard of the Law of Nations. It's an excruciatingly detailed treatise on the conduct of sovereign nations amongst each other where no nation reigns supreme over any other. It is believed to be the bedrock document defining the modern body of international law, or at least it is philosophically sound. Here are a few excerpts:

From Law of Nations, Book II, Chapter IV: “Of the Right to Security, and the Effects of the Sovereignty and Independence of nations.”:

In vain does nature prescribe to nations, as well as to individuals, the care of self-preservation, and of advancing their own perfection and happiness, if she does not give them a right to preserve themselves from every thing that might render this care ineffectual. This right is nothing more than a moral power of acting, that is, the power of doing what is morally possible,—what is proper and conformable to our duties. We have then in general a right to do whatever is necessary to the discharge of our duties. Every nation, as well as every man, has therefore a right to prevent other nations from obstructing her preservation, her perfection, and happiness,—that is, to preserve herself from all injuries (§18): and this right is a perfect one, since it is given to satisfy a natural and indispensable obligation: for when we cannot use constraint in order to cause our rights to be respected, their effects are very uncertain. It is this right to preserve herself from all injury that is called the right to security.

It is safest to prevent the evil, when it can be prevented. A nation has a right to resist an injurious attempt, and to make use of force and every honourable expedient against whosoever is actually engaged in opposition to her, and even to anticipate his machinations, observing, however, not to attack him upon vague and uncertain suspicions, lest she should incur the imputation of becoming herself an unjust aggressor." [Note: This is why we must wait for a legal verdict before declaring war on another sovereign.]

When the evil is done, the same right to security authorises the offended party to endeavour to obtain a complete reparation [Trump: China must pay $10 trillion to the world in reparations], and to employ force for that purpose, if necessary."

Finally, the offended party have a right to provide for their future security, and to chastise the offender, by inflicting upon him a punishment capable of deterring him thenceforward from similar aggressions, and of intimidating those who might be tempted to imitate him [“Peace Through Strength”]. They may even, if necessary, disable the aggressor from doing further injury. They only make use of their right in all these measures, which they adopt with good reason: and if evil thence results to him who has reduced them to the necessity of taking such steps, he must impute the consequences only to his own injustice."

** If then there is any-where a nation of a restless and mischievous disposition, ever ready to injure others, to traverse their designs, and to excite domestic disturbances in their dominions,—it is not to be doubted that all the others have a right to form a coalition in order to repress and chastise that nation, and to put it forever after out of her power to injure them.** "

As to those monsters who, under the title of sovereigns, render themselves the scourges and horror of the human race, they are savage beasts, whom every brave man may justly exterminate from the face of the earth. All antiquity has praised Hercules for delivering the world from an Antaeus, a Busiris, and a Diomede.”

and, from Chapter V:

Let us apply to the unjust what we have said above of a mischievous nation. If there were a people who made open profession of trampling justice under foot,—who despised and violated the rights of others whenever they found an opportunity,—the interest of human society would authorise all the other nations to form a confederacy in order to humble and chastise the delinquents. We do not here forget the maxim established in our Preliminaries, that it does not belong to nations to usurp the power of being judges of each other. In particular cases, where there is room for the smallest doubt, it ought to be supposed that each of the parties may have some right: and the injustice of the party that has committed the injury may proceed from error, and not from a general contempt of justice. But if, by her constant maxims, and by the whole tenor of her conduct, a nation evidently proves herself to be actuated by that mischievous disposition,—if she regards no right as sacred,—the safety of the human race requires that she should be repressed. To form and support an unjust pretension, is only doing an injury to the party whose interests are affected by that pretension; but to despise justice in general, is doing an injury to all nations.”

Finally, from Book III, “Of War”:

War is that state in which we prosecute our right by force.”

In treating of the right to security (Book II. Chap. IV.) we have shewn that nature gives men a right to employ force, when it is necessary for their defence, and for the preservation of their rights. This principle is generally acknowledged: reason demonstrates it; and nature herself has engraved it on the heart of man. Some fanatics indeed, taking in a literal sense the moderation recommended in the gospel, have adopted the strange fancy of suffering themselves to be massacred or plundered, rather than oppose force to violence. But we need not fear that this error will make any great progress. The generality of mankind will, of themselves, guard against its contagion,—happy, if they as well knew how to keep within the just bounds which nature has set to a right that is granted only through necessity! To mark those just bounds,—and, by the rules of justice, equity, and humanity, to moderate the exercise of that harsh though too often necessary right,—is the intention of this third book."

Were men always reasonable, they would terminate their contests by the arms of reason only: natural justice and equity would be their rule, or their judge. Force is a wretched and melancholy expedient against those who spurn at justice, and refuse to listen to the remonstrances of reason: but, in short, it becomes necessary to adopt that mode, when every other proves ineffectual. It is only in extremities that a just and wise nation or a good prince has recourse to it, as we have shewn in the concluding chapter of the second book.”

The right of employing force, or making war, belongs to nations no farther than is necessary for their own defence and for the maintenance of their rights (§3). Now if any one attacks a nation, or violates her perfect rights, he does her an injury. Then, and not till then, that nation has a right to repel the aggressor, and reduce him to reason. Further, she has a right to prevent the intended injury, when she sees herself threatened with it (Book II. §50). Let us then say in general, that the foundation or cause of every just war is injury, either already done, or threatened. The justificatory reasons for war shew that an injury has been received, or so far threatened as to authorise a prevention of it by arms.”

The immediate consequence of the premisses is, that if a nation takes up arms when she has received no injury, nor is threatened with any, she undertakes an unjust war. Those alone, to whom an injury is done or intended, have a right to make war."

From the same principle we shall likewise deduce the just and lawful object of every war, which is, to avenge or prevent injury. To avenge signifies here to prosecute the reparation of an injury, if it be of a nature to be repaired,—or, if the evil be irreparable, to obtain a just satisfaction,—and also to punish the offender, if requisite, with a view of providing for our future safety. The right to security authorises us to do all this (Book II. §§49–52). We may therefore distinctly point out, as objects of a lawful war, the three following—1. To recover what belongs or is due to us. 2. To provide for our future safety by punishing the aggressor or offender. 3. To defend ourselves, or to protect ourselves from injury, by repelling unjust violence. The two first are the objects of an offensive, the third that of a defensive war.”

When offensive war has for its object the punishment of a nation, it ought, like every other war, to be founded on right and necessity. 1. On right:—an injury must have been actually received. Injury alone being a just cause of war (§26), the reparation of it may be lawfully prosecuted: or if in its nature it be irreparable (the only case in which we are allowed to punish), we are authorised to provide for our own safety, and even for that of all other nations, by inflicting on the offender a punishment capable of correcting him, and serving as an example to others. 2. A war of this kind must have necessity to justify it: that is to say, that, to be lawful, it must be the only remaining mode to obtain a just satisfaction; which implies a reasonable security for the time to come. If that complete satisfaction be offered, or if it may be obtained without a war, the injury is done away, and the right to security no longer authorises us to seek vengeance for it.”

Defensive war is just when made against an unjust aggressor. This requires no proof. Self-defence against unjust violence is not only the right but the duty of a nation, and one of her most sacred duties. But if the enemy who wages offensive war has justice on his side, we have no right to make forcible opposition; and the defensive war then becomes unjust: for that enemy only exerts his lawful right:—he took up arms only to obtain justice which was refused to him; and it is an act of injustice to resist any one in the exertion of his right."

TL;DR: If one group of people unjustly attacks another group of people the ones who were attacked have an absolute right to seek justice through whatever means necessary including forming coalitions with other justice-loving nations to eliminate anyone posing a clear and present threat to the rest of the world.

DOES THIS SOUND FAMILIAR!?