The Holocaust, or shoah, that occurred in Nazi Germany during the years of World War II is a watershed event for Jews and Judaism in general. Approximately 6 million Jews - along with hundreds of thousands of others including homosexuals, gypsies, Jehovah's Witnesses, and political dissidents - were rounded up and murdered in extermination camps. The shoah is arguably the worst act of antisemitism in the history of the Jewish religion, and one of the worst human atrocities in the history of the world.
The horrors of this event completed the efforts of zionism to establish a permanent homeland for the Jews in their biblical land called Israel. Zionism - literally, a call or desire for Zion (which is another name for the biblical homeland) - began earnestly in the 19th century after Russian Jews endured pogroms and had to flee for their lives. Many began immigrating to the area now known as Israel, but which was then under Ottoman rule.
After the Holocaust, members of the international community that had been central the ending of World War II exerted the political will to establish a Jewish homeland. Thus, the modern state of Israel was established in 1948. Although Israel's founding included provisions for the founding of a state for native Palestinians in the region, such a state has yet to be established. Moreover, many Palestinians and other nations in the region rejected the establishment of the state of Israel for various reasons.
Therefore - and quite ironically - the state of Israel, which was founded to once and for all provide a safe place for Jews, has been a contested and sometimes besieged state since its founding.