mellowfruitfulness has the gist of it. Impossible to do much more without writing a large tome!
But it goes back even further than that. The Jews left/ were forced out in Roman times after a failed rebellion and destruction of the temple. They 'wandered' for centuries - the Diaspora - and were rejected from several countries and persecuted. they always regarded Palestine as 'theirs'.
Meanwhile, after a thousand years or so the area became part of the Ottoman (Turkish) Empire and remained so until the British kicked the Ottomans out in WW1. (See Lawrence of Arabia).
Some Jews went back then but the Brits had been allied to the various Arab tribes who had helped against the Turks and it all became a mess.
It got messier after WW2 when the concentration camps were liberated and the survivors of the Holocaust were determined to make a state of their own. The British tried to hold them back but they resorted to terror tactics against the British and it was impossible to keep the two sides apart. (The Promise shows this very well indeed.)
A poor settlement left Jerusalem partitioned and and Israeli state on the east bank of the Jordan river as well as a couple of excellent seaports. The Palestinians either fled or were pushed into the west bank.
In 1967 the Israelis went to war and in six days took the Sinae from Egypt and the west bank too. Israel had now hugely expanded and flexed its muscles, shown that it had a very powerful military and, most importantly, now had all the water because it controlled both banks of the Jordan.
Palestinians also resorted to terror tactics and the Israelis carried out appalling reprisals in refugee camps in Beirut.
Now the Israelis have all the best land, the water resources and military power whereas the Palestinians are penned up in barren land (two separate bits) with no access to port facilities.
Unfortunately there seems to be no basis for dialogue, no room for forgiveness and no chance of controlling the hardliners on either side. Both sides believe the land is theirs.
It's a religious and political conflict but it's also very much to do with control of economic resources.