In late 1947, their militias begun a campaign of massacring and expelling Palestinians from mostly defenseless villages. These refuges pouring into neighboring Arab countries is what prompted the 1948 war. When the war ended, they murdered any civilians trying to return to their homes.
Gaza was originally a refugee camp created for receiving these expelled people.
The ethnic cleansing and denial of rights has continued ever since. The current Gaza war is not when the crimes against humanity started. Israel has been commiting crimes against humanity throughout its entire existence.