Why does the world oppose Israel's possession of the Promised Land?

Because the world lies in darkness. Satan, the god of this world, has blinded the eyes of the people so that they simply cannot see the truth.

Another reason is that all the world will have to condemn themselves before the Judgment Seat of God when they must account for what they have done either for or against His people. Virtually every country has been established by force and violence, yet all the nations' borders are accepted in accordance with their success in using violence to establish their boundaries. They call it international legal borders recognized by the United Nations. The world makes an exception where Israel is concerned; not only do they consider the borders established by God Himself invalid, but they ignore the fact that Israel has also legitimately secured these borders through the action of war.