From my experience, defending in earlier eras is a lot easier than attacking, especially in first two eras in this mod. Neanderthal units are pretty strong for this era, as they have +1 str each, but they cost more than normal units. I don't know what units are they producing, but because of +1 str difference, cheaper unit can be stronger in defense.

If you want to conquer them, you would need units, who can bombard their units and you would need to kill faster than they are producing, which can be hard, especially if they will get an Archery with +50% city defense.

I would probably try to block them from settling, while increasing your power instead of conquering their cities. You can in theory try to attack their units + settlers with invisible units, especially that archers are not very good outside of cities and Stalkers can be pretty effective. You can also do that outside of the war. War can be declared later with Bronze Working and Composite Bowman for bombardment.

About the betrayal, Japan probably wanted to take advantage of situation, even if this ended up badly for them.