Actually only part of it. At one time "Palestine" comprised what is now Israel and Jordon (mostly). Then it was partitioned, with everything from the Jordon river westward to the sea designated for the Jews, with the larger part to the east of the river designated for the Muslims. When Israel declared statehood, the Arab Legion, which belonged to the current King of Jordan's grandfather, along with Syria, Egypt, Iraq and others, invaded and eventually took what is now known as the West Bank. Israel took it back in the '67 war, aka the "Six Day War".
See 7, the PLO has claimed all of it for over thirty years.