Here is my super biased, short summary view of the situation:
During WWI, England promised the Jews a chunk of land that the Turkish Ottoman Empire owned if they supported them financially. At the same time, they promised the Arabs the same land if they rebelled against the Turkish Ottoman Empire.
WWI ends, British keeps the land for themselves.
Jews and Arabs are pissy because they were both promised a shit ton of land and they both go terrorist apeshit on Britain and each other.
British get attacked by both sides, eventually give up and with the help of the UN, split the land in half.
The Jews, never having the land before, accept. The Arabs, always living on the land but never controlling it, try and seize the opportunity and reject the plan and declare war on the Jews, who now have their own country of Israel.
Due to distrust and disorganization on the Arab side, the Israelis beat back both the internal Palestinian region Arabs and the external Arab nations (who declared war on Israel shortly after its creation) and Israel take a greater chunk of land with it. The UN allows it because it falls within the UK/France's interest in the region. The Palestinian land (now roughly 20% smaller) gets temporarily absorbed by Jordan and Egypt.
The Arabs don't give up and border skirmishes are common--- Israel modernizes and bulks up with American made arsenal (at this point, America began getting chummy with Israel because the Soviet Union was chummy with Egypt). During a very big war where most of the Arab world went against Israel and still got their asses handed to them, Israel conquered all of the mandated land, plus land from Egypt, Syria and Lebanon. The UN is NOT ok with that one, it seems.
They give back the land in negotiations to eventually barter for peace deals (although the last war they fight almost toppled them over) but the Palestinians begin fighting for quasi independence and Jordan/Egypt give them up. This begins the occupation phase, as Israel cannot come to any peaceful terms with the Palestinians and just keeps the land out of security concerns. Eventually they say fuck it and annex a chunk of the land, and begin illegally building on the rest of it. At this point, what was originally mandated and what exists of Palestine today is more than half gone, maybe 1/3rd.
I'm sure many will disagree with my horrible shill summarization. I await the angry responses.