Dear Rabbi: The rabbi wrote: "If indeed it is a bona fide legal document, than he is the owner of that particular piece of land, and we are not allowed to steal grapes or even enter his land without his permission." If the rabbi would share the specific sources the rabbi used for coming to this conclusion and any other related sources of use on this topic, I would greatly appreciate it so I may continue studying this topic.

The source that you are not allowed to steal from a gentile is so obvious and famous (Shulchan Aruch Choshen Mishpat 348, 2) that you don’t really have too much to research. The fact that the State can and should conquer and confiscate is also very basic in the Tanach, but because it wasn't on the agenda for almost 2,000 years, we are a little "out of practice". See the entire book of Yehoshua; see Rashi on Breishit 13, 7, based upon Breishit Rabba 41, 5 on the difference between an individual (Lot) taking from a non-Jew (which was forbidden), as opposed to the national conquering which is obligatory; see Rambam hil. M’lachim ch. 4-6 and his commentary on the mishna Sota 44b; Ramban, addendum to Sefer HaMitzvot, 4, and on Bamidbar 33, 53; Responsa Yeshu’ot Malko, Y.D. 66. In Ma’amarei HaReiya, pp. 252-253, Rav Kook writes that we prefer fulfilling the mitzvah of inheriting and conquering the Land of Israel through acquisition, and war is just a last resort. With Love of Israel, Rav Ari Shvat