Well, if you want to talk about legitimacy... You have to set the rules and the terms of this legitimacy.
You can prove nothing unless you agree first on these terms. Dont play a game without setting the rules. If you dont want to, just start bashing each others like primitive men. And let the man with the bigger stick win.
But if you want to live like civilized men, first, talk to each others, and agree on some definitions, once you find a common ground, then you can state historical facts...
And I think it's not pertinent to say "Arabs have 30 nations, and we have only a few... so we can do what we want." That's not an argument.
Try to use that in something else. We dont see terrorists saying "you have 1000 tanks and we just have rocks and cheap rockets... Give us tanks." Well, we see some people saying that, but we cant understand that...
Now, we have to put racism aside, we all have equal rights, whatever the culture, the skin color, the religion... etc... Every human being have rights.
Some jews were born in the land we call Israel today, they have the right to live there. And nobody can tell us otherwhise.
And it's the same for the Arabs who were born there. It's their land too. They have the right to live in the land of their ancestors... etc... This is how it works...
Now, we have a situation where many people have the right to live in the same place... The problem isnt who have or dont have legitimacy, this is obvious.
The problem is in the cohabitation... They refuse each others...
What we need is an authority, capable of forcing these entities to live with each others... even by Kicking their asses if they dont keep it civil.
But who will do this job? This is the problem, nobody wants to be neutral...
We can solve the conflict very easily... The solution is obvious. But we dont have true people to do that. We are so corrupt, so blinded that we cant do what's obvious...