Sorry, but the League of Nations had nothing at all to do with it.
Basically it started with WWI, where the British promised Arab independence in exchange for help against the Ottoman Empire.
The Jews did not help so were owed nothing.
The Balfour Declaration just suggested facilitated Jewish immigration to Palestine, in order to boost is economy.
The International Zionist Congress that met in Carlsbad in 1920 and 1923, (in Hungary I think?), promised to want no part in politics or governing.
The League of Nations is not the source of any action or legal authority. The Treaty of Sevres and Treaty of San Remo are. And they created the British Mandate for Palestine, which meant England was responsible for getting Palestine's independence.
Europe and Congress pretty much all scoffed at the League of Nations idea, and only Wilson pushed it.
And the idea of not giving back something stolen only works if the victim does not know and can move on and forget.
The Palestinians not only are totally aware they have been ripped off, but are still constantly being ripped off, beaten, jailed, murdered, their homes stolen, etc. The whole notion of "Chosen People" and "Promised Land" is just totally and inherently evil, used to justify the worst imaginable crimes, and will continue to do so as long as Israel has weapons of any kind. They must be totally and completely disarmed.
And you need to learn some accurate history.
Zionists are the only aggressors.
If you want to understand who started the 1948 war, look up "massacre, Dier Yassin".
You will find that hundreds of Arab villages disappeared, starting around 1946, with all Arab inhabitants massacred.
en.wikipedia.org