The Arab-Israeli War started in 1948 because of the announcement of an independent Israel. With the announcement, five Arab Nations - Egypt, Jordan, Iraq, Syria, and Lebanon - invaded the region.
This resulted in the Civil War, known as the First Arab-Israeli War. The Israeli Defense Force expelled the Arab nations and expanded its territory beyond the partition that was made before, then Israel controlled most of the portion of the Mandate Palestine west of the Joran River.
This also led to a series of conflicts and crisis, such as the Suez War, and others.