In 1947, following increasing levels of violence together with unsuccessful efforts to reconcile the Jewish and Arab populations, the British government decided to withdraw from the Palestine Mandate. The UN General Assembly approved the 1947 UN Partition Plan dividing the territory into two states, with the Jewish area consisting of roughly 55% of the land, and the Arab area roughly 45%. Jerusalem was planned to be an international region administered by the UN to avoid conflict over its status.
Following the State of Israel's establishment, the armies of Egypt, Syria, Jordan, Lebanon, and Iraq joined the fighting and began the second phase of the 1948 Arab–Israeli War. From the north, Syria, Lebanon, and Iraq, were all but stopped relatively close to the borders. Jordanian forces, invading from the east, captured East Jerusalem and laid siege on the city's west. However, forces of the Haganah successfully stopped most invading forces, and Irgun forces halted Egyptian encroachment from the south. At the beginning of June, the UN declared a one-month cease fire during which the Israel Defense Forces were officially formed. After numerous months of war, a cease fire was declared in 1949 and temporary borders, known as the Green Line, were instituted. Israel had gained an additional 26% of the Mandate territory west of the Jordan River. Jordan, for its part, held the large mountainous areas of Judea and Samaria, which became known as the West Bank. Egypt took control of a small strip of land along the coast, which became known as the Gaza Strip.
Large numbers of the Arab population fled the newly-created Jewish State during the Palestinian exodus, which is referred to by many Palestinian groups and individuals as the Nakba (Arabic: النكبة), meaning "disaster" or "cataclysm". Many historians suggest that the Palestinians fled due to orders from Arab generals. Many Palestinians left under the belief that the Arab armies would prevail and they would return.