Our Gemara on Amud Beis discusses the ruling that testimony that one occupied a field or home for three years consecutively without any claims, it establishes a presumption of ownership. Yet this chazakah is only used to support a valid claim, such as, “I purchased this land many years ago and lost the bill of sale.” But if he makes no claim other than he occupied it with no challenges, even if he had testimony that he lived there for a hundred years, it would prove nothing as he is not claiming any prior means of acquisition. This is known as a chazakah she’ein imo ta’anah.
Chasam Sofer (Lech Lecha, ×´Acharei”) notes that Avraham acquired the Land of Israel legally via his war with the Four Kings to recover his brother, Lot, who was taken hostage. The Jewish people only occupied Israel hundreds of years later, nevertheless the nations that stayed in Israel meanwhile, had no argument that this was undisputed territory. This is because it is a chazakah she’ein imo ta’anah, evidence of occupation without a valid basis, which is insufficient legal proof.
In International Law, what Avraham acquired is known as Defensive Conquest. The idea is that this is legitimate is an inferred exception to the post World War II United Nations agreement regarding the Law of Conquest. The Law of Conquest was a centuries-long de facto recognition that territory acquired via war and surrender establishes legitimate ownership. This is universal law, that originally did not consider where the acts of war were legal or appropriately instigated. This is why America has not given back all of its land to the Indians and why other nations of the world have not changed historic borders established after their wars in the last few centuries. After WWII, the Nations of the World agreed that Conquest was no longer a legitimate grantor of ownership, and war itself is illegal. However, in the case of land acquired as a result of a Defensive Conquest, one may argue that it should be legal. It is a reasonable assumption that the original centuries-long law of Conquest should be activated, since a Defensive War is legally sanctioned. The potential hitch in this argument is that this logic may only work up to the defenders borders, but not annexation. Even so, any additional lands acquired in order to establish strategic security ought to be considered legal as well, given that they were acquired as result of legal defensive action. For an analysis Israel and territory it acquired via its many defensive wars, see
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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