Halakhah sur Téroumot 1:3

It is also possible for areas which were neither conquered nor settled in the time of Ezra, or even in the time of Joshua, to become sanctified as integral parts of the Land of Israel. Commenting on the verse, "Every place where your foot shall tread shall belong to you" (Deuteronomy 11:24), Sifre, Ekev 51, indicates that the Torah here declares that all conquered territories, even those lying outside the biblical boundaries of Erez Yisra'el, are endowed with the sanctity of the Holy Land. Thus, Rambam, Hilkhot Melakhim 5:6, writes, "All territories which Israel conquers by means of a king upon the instruction of the Bet Din … are for all purposes as the Land of Israel which was conquered by Joshua."⁷See, however, Radbaz, Hilkhot Terumot 1:3. Rambam, it should be noted, stipulates that a Bet Din (i.e., the Sanhedrin) is required in order to effect the sanctification of territory outside the boundaries of Erez Yisra'el—a factor which was clearly absent in all conquests of the modern era.

And [it] is practiced by males and females in the Land of Israel only, at the time that [the people of] Israel is there - as it is stated about it (Leviticus 25:2), "When you come to the land." And it is practiced rabbinically even at this time, only in the Land. And any place (Mishnah Sheviit 5:1) that those [Jews] that came up form Babylonia controlled until Keziv - but not including Keziv - is included in the prohibition of work, and all of the aftergrowth that grows there is forbidden to eat. As [these Jews] sanctified the places that they controlled forever. But in the places that those [Jews] that came up from Egypt controlled, but not those that came up from Babylonia - which is from Keziv to the river and to Amanah - even though, since they were stringent about [it], they are rabbinically forbidden today regarding work on the seventh [year], [nonetheless] the aftergrowth that grows there is permissible to eat; as it was not sanctified by those that that came up from Babylonia. And it is permissible even [for work] from the river and from Amanah and further. [With regards to] Syria, even though the seventh [year] is not practiced in it from Torah writ, they decreed that those places should be forbidden in work like the Land of Israel. And Syria is from the places that David conquered before all of the Land of Israel was conquered - and that is what our Rabbis, may their memory be blessed, called the conquest of an individual (Mishneh Torah, Laws of Heave Offerings 1:3, 9). And that land corresponds to Aram Nehorayim and Aram Tsovah, all along the Euprates to Babylonia, [including] such [places] as Damascus and Allepo and Charan and other places close to these. But the seventh [year] is not practiced in Ammon, Moav, Egypt and Shinnar, even though they are obligated in tithing (Mishnah Yadayim 4:3). And all the more so is it not practiced in the other places outside of the Land. And one who transgresses it and seals his vineyard or his field on the seventh [year] - or gathered all of his fruits into his house at the time that Israel is on their land - has violated a positive commandment. And nonetheless it is permissible to gather from them to his house a little bit at a time to eat - so long as the hand of everyone is equal in them, as if there were no known owners to the land.