What event sparked the call for the creation of a Jewish homeland in Palestine?

There was no particular issue that caused Zionism as a movement. Theodor Herzl, the founder of the movement, was motivated quite strongly by the Dreyfus Affair in France. Other Zionists were motivated by other rationales or events. The worldwide (Non-Jewish) community finally embraced the Jewish Right to a State after the realization of how extensive the Holocaust was.