You're not exactly a history student. So here it is.
The Brits ruled Palestine after WW1, but even during the war were puzzled exactly what to do with it, since it was basically filled with two warring factions. (You know which two.)
In general, even before WW2 -- even while the Brits were conquering the land in WW1 -- it was thought that there should be a Jewish state. But the Brits were not interested in enforcing their own decision, had been rapidly abandoning empire anyway, and post-WW2 had their own business to deal with. So they graciously agreed with the UN that the UN should handle it, in the name of the whole world.
So during 1947 the UN went off and had panels and whatnot, and came to the conclusion that the best way to manage this would be a
two-state solution with UN jurisdiction over the shared Jerusalem.
The entire world voted on it, and basically the entire world agreed, except for the Islamic countries. And so the Israeli state was created, but the Arabs were furious with the arrangement, refused to set up a state, and went to war with the new Israel on day one.
This then became the war for Israel's independence, and we call it that because they won. They expanded to take as much land as was thought to be needed to protect the new state, as it was thought the 1947 borders were indefensible. And the area that would today be a Palestinian state was then occupied (!!!!!) by Jordan (east bank et al) and Egypt (gaza).
Well this is how land disputes work. The UN has a go at it, they fail, and then there's a war. The winner of the war gets the spoils.