In Genesis 15:18, God promises Avram (Abraham) that his offspring will be given the land from the river of Egypt (it is disputed which body of water this refers to) all the way until the great river of Perat (commonly translated as the Euphrates River).
Is there an obligation for us to have sovereignty over those lands when Mashiach comes so that we will possibly have to go to war to acquire those lands, or will we be able to wait for Hashem to make it happen without having to militarily intervene?
(One can assume that the world will know that Mashiach is the true Messiah and that we will not have to make war at all, even if we are obligated to conquer, but it is possible that there will be some Amalekim or other enemies who will not accept him.
It is also notable that the verse in Deutoronomy 11:24--which talks about which lands we will conquer--makes no mention of the river of Egypt, but rather puts the 'sea that is behind' (Mediterranean Sea?) as the western border.
It may be possible that the descendants of Yishmael will be able to retain those lands because they are the offspring of Abraham. Even though Genesis (21:12) says that Isaac (Yitzchak) will be 'called offspring,' the next verse says that Yishmael is still the 'offspring' of Abraham, so when Yishmael's descendants fully return to Hashem, perhaps they will regain some of the inheritance.)