Complex sentence: It greatly increased popular support for Zionism, led to the creation of Mandatory Palestine, which later became Israel and the Palestinian territories, and was the origin of the ongoing Israeli-Palestinian conflict, considered the world's most intractable conflict.
Split into simpler sentences: It greatly increased popular support for Zionism, and led to the creation of Mandatory Palestine, which later became Israel and the Palestinian territories. As a result it is considered to have caused the ongoing Israeli-Palestinian conflict, often described as the world's most intractable conflict.