G-d has nothing to do with it. Some of the land was in fact given to the  
Jews by the United Nations, quite a bit of it was purchased from Arab  
absentee landlords. Present claims are based on prior ownership (purchase  
from aforementioned absentee landlords) award by the United Nations in the  
partition of the Palestine mandate territory, and as the result of  
defensive wars fought against the Egyptians, Syrians, Jordanians, et al.

***
First, I should point out that many Jews do not in fact agree with the  
idea that the West Bank is theirs. Since,  however, I agree with those who  
claim the West Bank, I think I can answer your question thusly: the West  
bank was what is called the spoils of war. Hussein ordered the Arab Legion  
to attack Israel, which was a poor move, seeing as how the Israelis  
promptly kicked his butt. The territory is therefore forefeit.  Retaining  
possession of ALL of the West bank is  not desirable, but it beats  
national suicide for the Israelis. Put another way, one could ask why it  
is that so many Palestinians seem to think that Tel-Aviv belongs to them  
and the future state of Palestine. As long as this state of affairs  
continues, it seems that to give the Palestinians a place from which they  
can launch attacks on Jews is a real poor idea. Giving up the entire West  
Bank would be idiotic froma security standpoint.  In addition, there is  
the small matter of Jerusalem, which is considered to be part of the West  
Bank. The chances of the Israelis giving up Jerusalem are nil. Even  
leftists who think Yasser is a really cool dude, like Yossi Sarid, aren't  
going to propose giving up Jerusalem. If he did, he'd get run out of town  
on a rail.
