The war broke out because Great Britain stopped administering Palestine as a mandate. When the mandate ended, Israel declared itself to be an independent state. The Arab countries did not want to accept the creation of a non-Arab state on what had been Arab land. Therefore, they rejected the UN's proposal that there should be two states made out of Palestine--one Jewish and one Arab. Instead, the Arabs (Egypt, Iraq, Jordan, Lebanon and Syria) attacked Israel and the war began. Israel won the war and was therefore able to actually come into existence as an independent country.