The Gemara discusses the concept of the acquisition of slaves via the spoils of war. Some of the commentaries understand it as an advanced form of yiyush, that is, the owners, seeing the overwhelming enemy give up and relinquish their hopes of reacquiring the item. Therefore, de facto, the conquerors take possession (see Rashi Bechazaka). From other Rishonim (see Tosafos 38a, “Aval,” and Rashba Kiddushin 14a) it seems that warfare itself creates its own form of acquisition.
There is something pragmatic and appealing about the Torah’s recognition that whether we like it, or not, warfare occupies its own particular form of morality. There is much discussion about reparations for various indigenous peoples whose land has been occupied. On the one hand, having relatives who perished in the Holocaust, and others who received reparations, it would seem hypocritical to reject the notion that certain displaced indigenous people are also not entitled to reparations. Perhaps this is self-serving justification, but it also has been indeed the way of all countries throughout the world, and throughout history, to generally remain in possession of land that was conquered during warfare. Post-World War II, there has been a change in perspective, and aggressive war is considered a violation of international law, despite the fact that, of course, it persists. Encyclopedia Britannica states (Entry: Conquest. https://www.britannica.com/topic/conquest-international-law)
Conquest is associated with the traditional principle that sovereign states may resort to war at their discretion and that territorial and other gains achieved by military victory will be recognized as legally valid. The doctrine of conquest and its derivative rules were challenged in the 20th century by the development of the principle that aggressive war is contrary to international law, a view that is expressed in the covenant of the League of Nations, the Kellogg-Briand Pact of 1928, the charters and judgments of the international military tribunals created at the end of World War II to try those accused of war crimes, the Charter of the United Nations, and numerous other multipartite treaties, declarations, and resolutions. The logical corollary to the outlawry of aggressive war is the denial of legal recognition to the fruits of such war. This implication was contained in what became known as the Stimson Doctrine, enunciated in January 1932 by U.S. Secretary of State Henry L. Stimson and subsequently affirmed by the assembly of the League of Nations and by several conferences of the American republics. The Draft Declaration on Rights and Duties of States, formulated in 1949 by the International Law Commission of the UN, contained (in Article XI) the rule that states are obligated not to recognize territorial acquisitions achieved by aggressive war.
The Ha’amek Davar comments on the following verse (Bereishis 9:5):
וְאַ֨ךְ אֶת־דִּמְכֶ֤ם לְנַפְשֹֽׁתֵיכֶם֙ אֶדְרֹ֔שׁ מִיַּ֥ד כׇּל־חַיָּ֖ה אֶדְרְשֶׁ֑נּוּ וּמִיַּ֣ד הָֽאָדָ֗ם מִיַּד֙ א״יש אדם לאחיו״
But for your own life-blood I will require a reckoning: I will require it of every beast; of humankind, too, will I require a reckoning for human life, of every person for his brother.
On the phrase, "A person for his brother”, Ha’amek Davar provides insight into the meaning of this verse. He explains that punishment is applicable when it is appropriate to act fraternally as a brother. However, during times of war and when there is a need to hate, it is a time for killing, and there is no punishment for that. This is how the world was established. As stated in Shavuos 45a, a kingdom can execute one out of six individuals without facing punishment. Even a king of Israel is permitted to engage in discretionary war, even though it may result in the deaths of several individuals from Israel.
War is not a pleasant phenomenon, and it is hoped that it can be avoided whenever possible. However, the gift of Torah morality and ethics is that it does not detach itself from real-world concerns. While we do not look forward to war and make efforts to avoid it, it seems to be a strong and deep pattern in human nature that there are times when conflict is necessary. War, apparently, has its own ethics.
There is only one thing more dangerous to mankind than war itself, and that is pretending that the instinct toward war does not exist within humans or that it will never be used. Ignoring human nature and pretending that there are no strong instinctive drives toward war provides little help in overcoming those instincts with rational thought.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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