Hi!
Really quickly:
Jews and Palestinians lived there side by side in peace, before WWII.
This is not true. We have narratives showing as far back as 1885 that there were clashes between Arabs and Jews, they had always had sources of tension in the past (including under the Ottomans in the 1840s for example), and riots occurred in 1920, 1921, 1929, and an Arab Revolt occurred from 1936-1939, and frequent clashes between. There was not a lot of peace going around before WWII, though it was less violent (most would say) before WWI.
When the jews got immigrated to the land of the later state of isreal, who owned it?