Are you asking what it was or when it happened? As to when, it happened after World War II, culminating in independence for Israel in 1948.
As to what, here are some details:
Jewish immigration into the Palestine region had been happening since the end of the 19th century and escalated in the early 20th century. The pressure for Jews to leave Europe escalated when the Nazis came to power in Germany. Then the Nazis went from persecuting Jews to a plan to annihilate them. The orders for the "Final Solution" (as they termed it) came in 1941 -- to kill all the Jews. The Holocaust was a systematic mass slaughter of European Jews and others in Nazi concentration camps. Adolph Hitler also commanded that all Jews in Russian territories be slaughtered when German troops began their invasion of Russia in 1941. The world became more sympathetic to the plight of the Jews after the horrors of the Holocaust became known.
After World War II ended, the United Nations (UN) adopted a plan for the partition of Palestine that would create a portion of that territory as the state of Israel. Arabs in the region and surrounding Arab nations were not in favor of this. On May 14, 1948, the Jewish leaders in the land proclaimed their independence as a nation, and a war with Arab peoples and nations in the region followed. Israel won that war and established itself as a nation. The new state of Israel was granted membership in the UN in 1949.
