So in 48 Jewish leadership accepted the partition plan and established a new state. The Palestinian leadership rejected it, and together with the neighbouring arab states waged war on the new Jewish state. The newly declared state was not defeated.

Go to 1967, and Israel wins a defensive war against its neighbouring Arab states (including the state that had control of the West Bank - Jordan). Israel wins the war, is not annihilated, and gains territory.

Basically, you think that a state that has gained territory in defensive wars, wars in which it's enemies have tried to wipe it off the map, should just give back the land it gained?

Out of interest, why didn't the Jordanians let the Palestinians establish a state in the West Bank between 48 and 67?