Egypt will not be able to stop Ethiopia from building dams on the river. The way forward is not for Egypt to try and stop the unstoppable. The way forward is to seek a win-win solution through diplomatic efforts.
So there is this likelihood that Egypt could start war.
Ethiopia and allies vs. Egypt & Sudan
No doubt Ethiopia and her allies have a just cause and therefore are the good guys and can expect the world's sympathy under normal circumstances. They are fighting against an unjust and strange colonial treaty written for the sole purpose of benefiting Egypt at the expense of other riparians.
It is not clear whether Ethiopia and the other upstream countries have formed a military alliance.
In the case of Egypt and Sudan we can be certain that there is such an alliance. It is also not clear how the AU will respond when a military conflict has begun. But we can be sure that the Arab League will back Egypt and Sudan. Though the Ethiopian group has the right cause the West and the international community may take a neutral stance overtly and support the Egyptian group covertly. You remember the negative response in the western media after the building of the Hidase Dam was announced ?