Colonialism and Right of Conquest was legitimate back when America was colonized. From the end of WW2 the World learnt that due to problems for people created by German Occupation of many territories that Right of Conquest would and was abolished. Therefore Israel cannot use Right of Conquest for illegal acquirement of Palestinian territory by force, especially when it ratified and signed up to the Fourth Geneva Convention that specifically outlaws Right of Conquest and illegal settlement building and transfer of occupiers citizens to occupied territory.
Western-backed Syrian rebels say kill foreign Islamic State (Reuters)
from the article: No, Moshe Arens, the one-state solution would be a nightmare